To begin your prep work, first get your piano ready:
Next, you'll tape everything off, sand, and then dust.
EDIT: * You can sand your piano indoors too. Just wear a mask if you have one, and vacuum and dust when you're finished!
For an amazingly smooth and lovely piano finish, don't ditch a good prep job. This includes taping, sanding, dusting, and priming. If you skip sanding it just won't look as professional as other paint jobs. It also creates an unbeatable smooth finish.
Using the brown craft paper and painter's tape, tape off and cover anywhere dust can get into, or anywhere you don't want painted. This includes:
Remind yourself that any color is better than a beat up color, and then begin to sand with 60 grit sandpaper. You don't need to sand until the brown finish is off. Just do a nice sanding over the whole piano. Sanding is used for two main reasons here, to (1) help the paint adhere better to the piano and (2) create an ultra smooth finish.
EDIT: If you don't have an electric sander that's ok! Just sand by hand real quick across the piano as best as you can. It will be better than not doing any sanding.
Wipe off most of the dust created by the sander initially with a dry rag, then use tack cloth to wipe the rest off. Using a tack cloth makes the finish super smooth and bump free. Wipe until it's smooth and without dust.