Offseason Free Agency works with blind bids. There are three steps
1. Make bids on FAs you want. All bids are the same length. The length of the bids is defined by your league.
2. The top three bidders on each FA can up their bid, or leave it alone
3. Winners are given a chance to sign a real contract.
Step One - Blind Bidding During this step you make your bid by clicking the $$ links beside players.
You can withdraw a bid, but once you do, you cannot rebid on that player.
Once you've bid, you cannot change the bid.
Step Two - Top Three Bidding You can either leave your bid alone, or up it. You can only change the bid once.
When you bring the player's contract screen, you'll see the other teams that are in the top three. They are sorted alphabetically, not by the size of their bid!
Step Three - Signing a Contract If you've won an FA, you can sign them a real contract as per your league rules. Your contract must be at least the same value per year as your bid. So if your 3-year bid was $15m and you want to sign a 5 year deal, the deal must have a total value of at least $25m.
Consult your league's Rulebook for any league specific rules regarding Free Agency.