Remove all the plastic parts you wish to paint from the DS. Sand the parts where paint will go with fine sand paper (at least 200 grit, preferably 400 or 600) until you've removed about as much material as the thickness of a couple layers of spray paint. Finish the sanding by wet-sanding (soak the sand paper in water- there is sand paper specifically for this) with 600+ grit.
Now, the painting.
Ish recommends spray acrylic. You can probably find Montana Gold at your local art store, but it's a little expensive. You can cut corners and use a plastic primer under regular paint (like Rustoleum Plastic Primer or Valspar Plastic Primer) like I do, but it's not the greatest for durability. If your DS is gonna be in pockets and backpacks and things, spring for the acrylic. You could also try Krylon Fusion for plastic, but it doesn't come in many colors and I've found it to be somewhere between the Montana Gold and using plastic primer + regular paint.
Whatever your choice, spray it on nice and even. Do at least two coats if it's a solid color. Try not to spray too close, or you'll get drips and you'll be sad. Finish it with a buncha coats of a finish, spray enamel or lacquer. That finish will be your finish, so if you don't want it glossy don't get clear gloss. Get matte or something. But don't skip the finish, or your piece will feel awful and be totally not durable. Trust me on this.
I like to put on a ton of coats of clear (like 10+) then wet sand it with a very fine (800-1000) sandpaper, but to each his own.
