My disclaimer - I am not a luthier but I spend a lot of my professional hours putting paint on wood and a lot of leisure time handling instruments. Here is what I can tell you, based on my experience and assumptions.
Your paint job will not look like a pro-job. If this is your aim, you may be in for a disappointment. It might seem minimal, but you are adding thickness to your surfaces and it may change the sound of your instrument. That being said, you are going to need to sand it first. This will allow the paint to stick to the surface. I wouldn't use anything heaver than 220 grit. You will need to mask your neck/fretboard, sound hole, and find a way to suspend the instrument so you can paint all sides. You will want to go with multiple light coats, sanding each with fine grit sandpaper. Finally you will want to clear coat it with several coats using a super fine steel wool between coats.
There are some youtube videos that will help you more, but I hope to let you know what you are looking at effort and materials wise.
All of this being said, Mike DaSilva teaches classes using a hand painted (by brush) ukulele. If you aren't familiar with what he builds, check him out.
Customized instruments can be a source of great happiness and pride. I look forward to seeing your ukulele makeover.
