Find a design you like online, or just make one up. I like to have a visual on paper before I start.
Use a thin paint brush to paint the design onto the device. I needed to do two or three layers before it looked nice, and I used a toothpick to correct tiny mistakes.
Once the paint has dried completely, apply a thin coat or two of mod podge. I painted over the battery-door thing, so I removed that to mod podge it, so that it wouldn't be stuck together.