In short, you had to sign up on the Rockstar Social Club website and link your PS3 or XBOX360 ID and then complete the game to 100% before 5/19/08.
There ended up being some bugs and glitches with the contest, mainly issues with linking the Gamer ID to the site and it retrieving the correct information.
Just to give you an example, when I look my ID up in the database if shows me joining the club on 6/07/08 even though I completed 100% with more than enough time to spare before the contest ended. Props to rockstar though for keeping their word that they had the correct information regardless of what the site says and here are some pictures of the just prize that I dedicated my 40 or so hours for.
They just finally got them mailed out to everyone that won, just short of 6 months after the end of the contest.