So you want to write a Cloudcuckoolander. The crazy guy (or girl) who lives with their head in the clouds, or never seems to get the point, or just seems somewhat odd. You know them when you see them and for the most part other people do too. But they're awfully hard to write without going overboard on the crazy and making them look insane, or just stupid, and you're here asking how to write them.
But first of all, why do you feel your story warrants such a character? Most likely you want a form of Plucky Comic Relief in your story. First of all that requires YOU to have a sense of humour. But not all genres need a Cloudcuckoolander. They don't work with all genres, and if pigeon-holed into the wrong kind of story, you might end up spoiling your entire story.
First of all, don't forget that all the things that make an Original Character good, also apply to the Cloudcuckoolander. You can't skip basic character creation classes just because you're creating a character who's not like all the others. So go and learn how to make an Original Character first, and then come back here for the fine tuning.
Your main issue is with making the character weird, but not insane. The Cloudcuckoolander isn't crazy; they just operate on a different wavelength. They see the world differently. It's the old stereotype that while we're working on Windows, they're using Linux. To write them successfully, you need to be absolutely sure, firstly of what the conventional world view is, and secondly of what your ensemble's attitude is. After all, if the whole group is 'crazy', a Cloudcuckoolander has to stand out in a different way.
It cannot be repeated enough times: Everything a Cloudcuckoolander does makes sense TO THEM. Their crazy ideas or actions are not just offhand attempts to get attention. Some Cloud Cuckoo Landers don't even like attention. But they get it anyway. They don't go jumping off bridges for the sake of it.
How exactly IS your character weird? Because there are so many ways for it to happen. They can be prone to Comically Missing the Point, or coming up with insane plans. It may be accompanied by something as simple as a Nervous Tic they are totally unaware of but other people do notice. Heck, they may be the Only Sane Man in a cast of idiots, and mocked for it.
Some Cloud Cuckoo Landers just plain speak their mind all the time, perhaps wondering what's for breakfast in the middle of a Zombie Apocalypse, and then confused as to why everybody's looking at them funny.
Too many ways to be weird risk making them just look stupid and aimless, or worse, totally insane even though that wasn't the point of the character.
Not enough, and your character is just another Regular Schmoe, but you're drawing attention to them for some reason your readers can't fathom. This results in them possibly becoming The Scrappy or worse, a Mary Sue.
Points To Remember
Your Cloudcuckoolander is not totally, off-the-wall, insane. Their ears hear the conversation. And somebody - sometimes somebody quite bright - is home and taking it all in. A true Cloudcuckoolander is not the result of missing part of a situation. And if everybody is crazy, that's another place altogether, and is more likely to be Cloudcuckooland.
In a situation where we would have no choice but to wait helplessly or throw a rage fit, the Cloudcuckoolander will choose option three - taking action. Chaos is not merely something that they enjoy, but they are in fact a magnet for it. They crank everything Up to Eleven.
It is important to remember that a CCL doesn't know what they've done wrong. They cannot quite fathom that they have done something against the norm. They can see everybody else as being sane and sensible, but then again, so are they. Why shouldn't you wear a dead man's boots? He won't miss them. And what's wrong with turning your messy flatmate's room into a landfill anyway? Sometimes of course, they think they're the Only Sane Man, but this happens rarely and when they do, it is markedly a point to emphasise how chaotic they are. They are more likely to think you're BOTH right.
The Comedic Cloudcuckoolander
The comedic Cloudcuckoolander does not often crack jokes. They are not deliberately trying to be funny. Instead of cracking jokes, they ARE the joke. They are often a Butt-Monkey for the show, and yet it is their reaction even to recieving the abuse of the entire universe that makes them so funny, not what happens to them.
The comedic CCL is played in a lighthearted manner. Their most likely reaction to doing or saying something odd is not, "Whoops, my bad" but "What did I do wrong?" Alternatively they may not even notice that the world is taking a dump on them for being different.
One excellent example of a Comedic Cloudcuckoolander is Phillip J. Fry from Futurama. Almost everything he does is supposed to make you laugh, but the characters surrounding him are frustrated by the way he always seems to miss the point. He certainly doesn't intend for people to laugh at him, and he doesn't really make any more jokes than any other member of the cast. It's the crazy things that happen to him, and the ways he responds, that make him so funny.
The Dramatic Cloudcuckoolander
The Dramatic Cloudcuckoolander is more likely to be a Deadpan Snarker, or to think of himself as the Only Sane Man. But once again, it's not the jokes they make, but their reactions to the world around them. They have more backstory than the comedic CCL; there's usually a reason they are so weird.
In a situation where others feel helpless, the dramatic Cloudcuckoolander as usual takes action. Messy, violent, and irrational, but it is action regardless, and because there's no other choice, other people go along with it when they would normally have done nothing but sit and wait.
A dramatic CCL can really plan, and they'll think up the wildest plan ever. Then they'll be surprised when it doesn't work, or if everybody else tells them where to shove it. Or if it's a plan that's reluctantly used, they won't think it odd that it's being used. Or if it does work out, everybody else will be amazed it worked, but they'll usually not be surprised at all.
An example - Alice falls off a building. In desperation, Bob fires a grappling gun at her, and it spears her leg, but her life is saved. What's the first thing she does? Feel relief at being saved? No way - while we would scream and wail and cry on the other end of the rope, waiting helplessly to be pulled up, CCL Alice has chosen the third option - grab her cell phone, and call Bob while hanging perilously inches from death. What does she chat about? It doesn't MATTER. She has done something that both has roots in reality but none of us would think to do - until now.
Male Or Female
The ratio in fiction is about equal, but your readers will expect different things from different genders. As well as all the other crazy things, a CCL will often wildly deviate from the norm of their gender, whether with dress code, hobbies, or general decisions. The male CCL is the first to admit that the female clothing they have to wear actually feels kind of comfortable. The Female CCL on the other hand is often The Ditz.
It is the why, not the what, that truly defines the Cloudcuckoolander. They're not doing it for attention. They're doing it because it's what they want to do. There can be a CCL on either side; whether good or bad, they just don't see the world in the same way as their co-workers.
When everybody else goes nuts in a panic, the CCL ignores the panic because something else is on their mind. Or maybe they do panic, but it's for a completely different reason. In a Zombie Apocalypse, the CCL is more likely to worry about running out of potato chips than they are to worry about running out of ammo.
The CCL is chaotic in every situation, but that doesn't mean they drop their pants for no reason. Sure, they may drop their pants, but there must be a reason that the reader is aware of, even if the other characters never work it out.