Step 1: get a big hat. Step 2: lick electrical sockets.
Springing from the Only So Many Canadian Actors phenomenon, Six Degrees of Canada, also known as Canadian 6 Degrees or C6D, is a multifandom fandom that grew out of the Due South community. Originally, it was known as "Six Degrees of Due South" and included fanfic based on other projects that the actors from Due South worked on. As it grew and expanded the term C6D was coined (most likely in 2005) to describe the wider range of fandoms now encompassed by the larger Six Degrees fandom.
Despite this, the fandom still largely revolves around works involving actors who were regulars in Due South, especially Paul Gross, David Marciano, and Callum Keith Rennie. It's generally safe to say that if an actor was in Due South, Wilby Wonderful, or Men with Brooms, then they're considered a C6D actor and anything they've been in is considered C6D (though in the case of tv shows it often doesn't "count" unless it's a recurring or particularly memorable role). Thus, for example, Inception would be considered C6D by some as it has Elliot Page in it. However, Inception and other Hollywood blockbusters are rarely cited as C6D and don't typically get much interest from the C6D fandom as such; independent and/or Canadian films/tv shows tend to be the hallmark of C6D media.
There is also a good deal of crossover with Hockey RPF and other Canadian media and things that trade heavily on themes associated with Canada, naturally. An attempt at a comprehensive list of possible C6D fandoms can be found here, though it has not been updated since January 2011 and leaves out some of the work by American actors in Due South, such as David Marciano's role in Homeland.