Fantasy TF2 is a fantasy sport game for Team Fortress 2 competitive leagues. Players of Fantasy TF2 can assemble a team of players by spending money from their fantasy budget. Those players then score points for your team from their in game actions when they play in the league. If someone is underperforming you can sell them and bring in somebody else, or make tactical transfers to take advantage of scheduling to maximise your points scoring opportunities.
Sign in using your steam account via the Sign Up link on the homepage (you can't miss it). Then choose one of the active leagues to create your team in. On the following page you'll be able to select your team roster (class restrictions and number depending on the league format) within the contraints of a budget. You won't be able to load up with star players, so careful consideration should be given to your selection and smart compromises made.
Each league will have a restricted number of changes you can make over a 7 day period, the default for 6v6 leagues being 2 players a week. On the homepage click the edit link next to your team name and you'll be taken back to the team creation page to make changes. You can juggle players around and take back changes you've made until you hit the confirm button, but you'll always be limited by the number of changes you have left in that 7 day period. Your budget never changes from the original amount and the players values will never change. This may change in the future.
At the moment you're playing for the honour, as I don't think it would be entirely fair to take people's money for a high rollers version on what may be a buggy system. If I get a sponsor who can offer prizes then of course I'll plug those right in.
Because enforcing changed classes would invalidate existing teams and cause all kinds of interface problems they'll continue to be bought and sold as their original class. They will still get points because it doesn't matter what class you play, your stats are your stats. It's up to players to decide if they still represent value on their new class.
Transfer locks prevent team editing during matches to ensure that players aren't moved around during sensitive periods and won't get the wrong score when the matches are processed.