I haven't taken the 21 bumper off but I did a lot of work to my 18 and it's very straightforward. You do not have to remove the headlights which is a huge plus.
It's best to start under the car, remove the plastics, and you'll see a handful of fasteners. The inner fender liners need to be loosened up since there are a couple of fasteners there too. Then under the hood, the fasteners are very visible. When removing the bumper, you have a couple of Molex plugs for the sensors so be careful not to drop it without disconnecting them.