Very simple...
1. Go into Free Agency Section (you already did, great job!)
2. Create thread. In the thread title it should be the position of the player and then the players name. Ex: "P Ben Sheets"
3. In the thread, post your team, your waiver # and that you want to place a bid. Ex: "Cubs (2) place bid on Ben Sheets"
4. WAIT! If no one with a lower number bids, your free agent may be added to your team within 24 hours of the initial bid.