The bumper is easy to remove : few screws under the car, 4 ones on the to next to the grill and an bolt behind the wheel arch. And disconnect the fog lights ...
For the sideskirts, open the doors, remove the screws in the wheel arch, measure the holes position in the body shell, install the skirts, drill them for the screws to come back in place. Fix it with double side tape under the doorsills. Put the screws back on.
You should have fitted it before painting as it surely needs modification (cutting, drilling, sanding ...) to fit if it isn't a Toyota /TRD prepaint bodykit.