"... all my tales are based on the fundamental premise that common human laws and interests and emotions have no validity or significance in the vast cosmos-at-large." That quote tells you all you need to know about the mindset of the author you've chosen to emulate. Something of a misanthropist, HP Lovecraft's tale revolved around the idea that life really was ultimately meaningless; human experience and belief was worthless, human religions feeble artifices, and if there were any Gods out there, then they were either so far beyond us that they literally showed no interest in whether we live or die, or are actively malevolent and wish to consume us and torment us for all eternity. Optimists in this universe are not welcome and not likely to do so well. Lovecraft's stories jump-started a modern horror movement focusing on Eldritch Abominations waking from their eternal slumber to torment humanity, who is entirely defenseless and pathetic against their onslaught. If you choose to write in this manner, and have familiarized yourself with the ancient, arcane rules that govern the telling of stories, then read on. But step carefully, my friend. To lose your footing in this genre is not merely to lose your mind - it is to lose your soul.
Necessary TropesThe concepts that underlay the Cosmic Horror Story should form the backbone of your work. It doesn't even matter whether you have monsters in your story or not (although that said, Lovecraft's works aren't exactly short of them); much of Lovecraft's horror strikes at a more existential terror than mere horrors under the bed, raising the uncomfortable question of whether there is any point to life whatsoever; if there's nothing after death, and human endeavour is ultimately meaningless, and we are simply insignificant specks of dirt in a cold, unforgiving universe that, if it's not actively trying to kill us, isn't interested in us at all, then why even bother? It's not just a World Half Empty Lovecraft inhabits; it's a Universe Half Empty. Of course, Lovecraft's most influential legacy is the Eldritch Abomination - the Great Old Ones, the Elder Gods, who slumber eternally in wait for the time (which may be nigh) when the stars are in alignment and they can wake to sweep over the Earth once more, brushing humanity aside like lint. The important thing about the Eldritch Abomination is not simply that they are evil or uncaring, or that they are big ugly monsters with tentacles; they are other. They are so beyond our comprehension and understanding of how the universe works that to look upon them is to go insane. Lovecraft gave us some of the more famous ones - Cthulhu, Dagon, and other horrors that must not be named - so if you want to homage the master, these will probably appear somewhere. They will almost always be Sealed Evil in a Can from The Dark Times, lurking deep Beneath the Earth, waiting until the stars are right in order to rise again. Lovecraftian Fiction also comes with numerous tropes designed to squick us out and plunge us deep into a universe that not only operates against our understanding of physics, but against any kind of logic at all. Body Horror is common — the fate of many in Lovecraft's works and his imitators is to be transformed into something entirely inhuman, a grotesque parody of man and nature. (Or, in a transformation that can be even more horrible if it's done well, something that is not quite entirely inhuman. Just enough traces of humanity remain to maintain identification.) Many of them are also set in Lovecraft Country or Campbell Country, often miserable, squalid out-of-the-way places where ancient rituals and dark arts can be practiced in relative isolation. The Tome of Eldritch Lore usually also figures prominently. Also, the Downer Ending often comes into play. Lovecraft's stories rarely, if ever, end well. The whole point is that human endeavour is futile; the coming of the Great Old Ones is as inevitable as death itself. Your protagonist may win a minor victory, but the emphasis will be on 'minor'; it will be a setback to the antagonists at best, and will have come at great cost to the protagonist; if they're lucky, they will merely Go Mad from the Revelation. At worst... well, let's not talk about that. It should be noted that, whilst the Cosmic Horror Story and the Eldritch Abomination are probably Lovecraft's most well-known themes, they weren't the only things he focused on; several of his stories are rather more mundane in focus, for want of a better word. For example, Herbert West: Reanimator is a prototype zombie thriller, The Picture In The House deals with cannibalism, The Tomb with madness, all with little in the way of Cosmic overtones. Cosmic Horror Story aside, it could also be argued that Lovecraft's other central focus was the nature of insanity, making him something of a pioneer of psychological horror as well. As mentioned above, many of his protagonists end up going mad; however, in several works, they either battle those who are already mad or are themselves suggested to be in less-than-stellar psychological health to begin with, thus making us question what, if any, of the story we've just heard is true. Descent Into Madness and Through the Eyes of Madness form a significant part of Lovecraft's work as well.
Choices, ChoicesIndividuals interested in writing Post-Lovecraftian fiction might check out this essay. What Is Post-Lovecraftian?. A discussion of common Lovecraftian motifs and ideas can be found here. Musings on Lovecraftian Horror
PitfallsIt has been argued by some that Lovecraft was a great writer in spite of, not because of, his writing. Whilst his stories are memorable and engaging, they are also packed with a lot of issues, problems and flaws that less-stellar (or more overtly imitating) writers can often be trapped by. Lovecraft has been homaged by many; the so-called 'Cthulhu Mythos' was developed mainly by other writers using Lovecraft's works as inspiration. This means that his works have been parodied, homaged and imitated by countless writers over the years, so if you're going to attempt to follow in his footsteps, try and find something new and distinctive to say about it. Be wary also of Purple Prose. Lovecraft used it to excellent effect, partly because he was knew what he was doing; other writers who have imitated him over the years have been a bit less confident, meaning that some of them come across as more laughable than chilling. Such as, probably, this guide. Remember also that Lovecraft is frequently bleakness taken up to eleven; hope is usually not welcome in his stories. However, very few writers are capable of making the plunge into the dark depths that he managed to reach. Remember also that a lot of Lovecraft's stories must be read with a great deal of Values Dissonance in mind. Unfortunately, Lovecraft was quite racist, and many of his works are not exactly subtle in their comparison and association of Lovecraft's celestial horrors and disfigured monstrosities with particular ethnic groups, and must be read with that in mind. Following Lovecraft's path that closely may result in unfortunate associations. Film adaptations of Lovecraft's work have employed far more violence and sexual content than should be possible for the source material. Don't make the mistake that these people made.
Potential SubversionsThe most common subversion is when the omnipotent, all-powerful Eldritch Abomination turns out to be neither as omnipotent or all-powerful as it's reputation would have us believe. This subversion often appears in works which borrow Lovecraft's trapping but generally aren't set in a Lovecraftian Fiction universe (soft fantasy or science fiction, for example). Depending on how this is done, it can puncture the extreme cynicism of Lovecraft's universe... but not necessarily. Think about it; if a cold, unforgiving universe inhabited by uncaring Eldritch Horrors is bad enough, a universe that is so cold and unforgiving that even the Eldritch Horrors ultimately don't stand a chance would be infinitely worse. Another, more optimistic subversion would logically be that there is hope, after all, and that things are not as bleak or unforgiving as Lovecraft would have us believe; again, this is a drastic step away from Lovecraft's essential mindset, but it's also important to remember that few - whether writer or reader - are ultimately quite as bleak as Lovecraft was. Lovecraft didn't sell well during his lifetime. Many Lovecraftian tales also take place in the dank, creepy environments described in Lovecraft Country or Campbell Country - it might be possible for you to envisage Lovecraft's mindset and cosmic horror taking place in another environment altogether. Also, you might want to try some variation with the human characters. Most of Lovecraft's protagonists were introverted, academic/intellectual types, not unlike Lovecraft himself, if they had any characterization at all. You might want to try seeing how different sorts of people react to Lovecraftian Fiction situations. A good example would be the acclaimed radio series Quiet Please, which featured many stories with Lovecraftian themes from a much more blue-collar perspective. You could potentially have a student of marine biology/ non-elucidean mathematics tell your creature that it's not scary, and have the monster complain that it's only trying to do their job and /or run off crying that it's a failure (admittedly, this would probably make you the next Terry Pratchett).
Suggested Themes and AesopsThese Are Things Man Was Not Meant to Know was Lovecraft's big obsession, and his stories ultimately demonstrate that some avenues of knowledge should not be explored; that there are things about the universe that we really are better off not knowing about. Indeed, humanity needs to not-know these things in order to be able to effectively function; You Are Not Ready for this knowledge, The World Is Not Ready for this knowledge and it will destroy if you and us if it is explored too closely.
Potential MotifsThe weirder the monsters look, the better, unless you're going for an Uncanny Valley effect. By the same token, Starfish Aliens speak Starfish Languages unless they've learned other languages from humans. If you can turn your audience's expectations away from Aliens Speaking English, the horror will be greater when you finally use it. corrupt religeous imagery. Swastikas and pentagrams, both originally occult symbols before Those Wacky Nazis and Heavy Metal co-opted them, feature prominently in Lovecraft's works.
Set Designer / Location ScoutLovecraft Country and Campbell Country are the standard settings. However, the desired effect can also be found in any kind of out-of-the-way place. Wherever you set it, you're often best served if there's an atmosphere of decay, corruption (physical and moral) and oppressive, inhuman conditions.
Props DepartmentThe Tome of Eldritch Lore, which describes the Eldritch Abomination and the many horrors and depravities associated with it, is generally a must. It may or may not actually be alive.
Costume DesignerThe characters' costumes depend on the setting. Specifically, their clothes should match whatever academic field they're in—if they're Mad Scientists, they should wear lab coats; if they're Antarctic explorers, dress them in thick furs; cultists get Black Cloaks; for any other profession, old-fashioned, ill-fitting suits will do the trick.
Casting DirectorFusty, obsessive and reclusive academics who spend most of their time poring over dusty, ancient tomes in dark, secluded libraries intently studying the aforementioned Tome of Eldritch Lore form a key part of Lovecraft's casting. Many of them are not that well-adjusted prior to their encounter with the Eldritch Abomination. The Mad Scientist who dabbles in that which really should not be dabbled with also tends to make an appearance. Cultists also show up, so a Sinister Minister type could be used. People who are in the habit of muttering to themselves under their breaths work wonders for these guys, just put them in Black Cloaks.
The Epic Fails