How do you fly in Team Fortress 2

You can't fly in Team Fortress 2. You can, however Rocket jump and Sticky Jump. Apply explosives (e.g rocket launcher, stickybomb launcher etc.) under your player to launch him in the air. This method is commonly used in Team Fortress.
You can also use the noclip server command, if sv_cheats is set to 1. This will turn collision off and you can move in mid-air.