Opening The Controller
There's seven screws You need to remove inorder to get the controller open. Six of the screws are visible, however one of the screws are in hidden behind just below the charging gold terminals. The sticker should have a long product number.
Washing the Controller Pieces
Its important to wash the controller parts inorder to get the oil from your hands or any dirt. Just take all the parts that you are going to paint and wash them and be sure not to get any of the electronics wet, so just take the shell.
Preperation
Now for taping off the controller buttons. This keeps the controller buttons from sticking when pressed. Create an oval of tape on the inside of the controller buttons (A,B,X,Y,Guide Button, Start and Back, Analog Stick Holes, and Dpad). Make sure your tape is even because just as a reminder there will be no paint behind the tape so even if it is a little off it will be noticable. This part will be the most time consuming. Once you have all that done you are ready to move on to the next step.
Priming
Now its time to start priming. You are going to want to put at least 3 layers of primer. Start your first coat off nice and thin making sure you spray past the controller. You can light sand it in between coats with high grit sandpaper like 600 - 1000 to make it smoother.
Painting
Now for the paint. Apply the paint the same way as the primer except go a little heavier with it on the first coat. Again you can sand in between coats with high grit sandpaper.
Acrylic
Now the acrylic is the most important layer. This layer prevents the oil from your hands from destroying the paint. Spray this on just like the primer and I would put at least 5 good coats of this.
Afterwards
Let your paint cure for about a week and reassembly your controller.