You can find the Google doc of it here, along with all relevant information on where to send your letters and how to get them featured.
I think Google should start promoting ponies at this point with how much we use their document system.
I'm not sure if this can be considered fanfiction or not, so I'll just roll with it being not fanfiction until someone complains!
Also a Letters to Rainbow Dash Facebook page!
Here is a copy paste from letter brony!
"I'M A SPACE MONSTER GRRR"=BAD
PONY TO PONY=BAD (seriously, I only did it that one time to open up a storyline, and that was before the ED post existed. I might do it once more.)
LETTERS THAT ARE THE EXACT SAME AS OTHER LETTERS=BAD
LETTERS THAT THROW OUT EXISTING STORY FANON=BAD