Steps

Identify what type of controller are going to be used. The ones I’m reverse Engineering are PowerA Scuf Xbox controllers.

Unscrew 6 screws on the back plate to the controller.

Unscrew 2 screws located on the left and right side. 

To remove the Bumpers, you'll need to move this plastic then you can just pull it off.

To Remove the Triggers, you'll first need to dislocate the joint on the backside of the controller by pulling both sides apart. then turn back to the frontside to unclip the plastic off the circuit board. Then you could just pull it off.

To Remove the Joysticks, D-pad, and Motors, you'll need to desolder them when you can take them off the circuit board.