Basically, you wait for a GM to start recruiting (watch out for threads labeled [recruiting] or something like that) and post that you want in.
Alternatively, you could look at some already running PbP's where player activity may be low and contact the game's GM to see if s/he's looking for new players.
(Or, hey, run a game yourself! Just start a thread for recruitment, players will sign up!)
As for the actual playing, well, GM's post what they'd normally tell players, players post their characters actions. Rolls are usually done through invisiblecastle.com. For examples of play, just read some of the IC (in character) and OOC (out of character) threads.