CA, Earl's correct, there is tons of great info in other threads, but I'll take a moment to mention a couple of notes here. 3-2-1 applies to spare ribs, baby backs are closer to 2-2(maybe 1 1/2)-1. With either method, the first # is variable by quite a bit based on pit temp and the age/fat content of the ribs. The 3 period is over when the meat begins to pull away from the ends of the bones by 1/4"- 3/8". I always spray generously when wrapping (I use apple juice or better yet juicy juice brand apple juice concentrate).
3-2-1 enjoy!!