For the painting process you need to make sure your outer casing is clear of components and the rear glass panel as spray-painting might damage them.

For spray painting cover the insides with tape, which makes sure the components could fit when you put them back. Then, hang the casing up by a loop of wire. Apply the paint by spraying around the frame of the casing. Be careful to apply the paint uniformly and not so thick.

Because I'm still a middle school student naturally I can't get my hands on spray-paint cans and all that. I'm just using paint pens on this XS. They work just as well but you have to be careful to apply the paint uniformly.