First up, the materials:
Step 1: Opening the Headphones.
This involves putting the headphones over hot water in order to heat the heat-sensitive glue. After a few minutes, just pull them out.
Separate the cups from the driver housing.
Step 2: Painting.
Before painting, make sure to give the cups a decent clean with methylated spirits or other cleaning agent.
Spray lightly and evenly. I used 3 coats of paint.