^ wat Rob said.
But heres a general overview (I personally think we NEED a PAINT ONLY section for the DIY just for this kinda stuff
1. Start by degreasing whatever the part to be painted is using a PLASTICS DEGREASER. Reason I say use the specific type is because with ABS and other auto plastics, there is a mold release agent that stays on the surface form the factory and it will not come off with your average degreasers. Look for specific plastic cleaners for the best results, they work wonders and I have seen jobs go wrong because they weren't cleaned properly
2. Once thats done, you can start your sanding. Grit wise will depend but any microgrit will work so Id stay with 240 and finer. You'll know once you start going and the texture begins to change. Some plastics wont need any, some a little and others more. It all really depends on what finished texture you want the surface to have.
NOW, you said your uncle is doing the painting, if thats the case then he can take it from here because all the spraying needs to be done simultaneously. If I were you, Id go buy a plastic adhesion promoter from your local auto paint supply store or any automotive store for that matter. (your uncle will lay this down before the base)
Also get all the paint you need but be aware that when painting plastics, normal automotive paint isn't going to work to well on its own and will need a flex agent because the plastics flex and if you don't add some to the base/clear (if applicable) then it will crack and peel down the line. Some paints/clears come ready to spray but you should check with the mfg.
Best bet is just to get all the stuff and have your uncle lay it all down, he probably knows the deal since you said he paints so shouldn't be too new for him. Best bet when looking for instructions is to call the mfg's of the products, they can usually tell you best what to add and how to apply it exactly for best results, I mean that is the business they are in
Short recap, heres what you need:
Plastic Adhesion Promoter
Paint (made for plastics) OR
Paint that you can add some flex agent too if it isn't designed for plastics
Clearcoat (same deal as paint, they make ready to spray and then there's your regular auto clear)
Besides this you just need to spend some time and keep reading up. Not to hard but many people have bad results with plastic just because they rush and dont clean/sand enough and then their application is completely wrong for plastic as you can spray the same types of stuff you would on sheet metal and expect the same results haha. G/L with your job.