If you want to do flat black, its pretty easy, i prefer gloss though (but orange peel is a killer for inexperienced peeps like me)
1- sand it down, as much as possible to get the old finish off start with a lower grit like 220 and work up till about 800, no higher as it tends to stop the new paint from sticking.
2. give it a good wash and use some wax/grease remover. I use white spirit cause im cheap. :P
3. give it a few coats of your choice of primer, i usually use etch primer on most things just because i tend to sand or strip to bare metal. ive never had a problem with paint cracking or peeling off plastics with etch primer but you can also use a plastic primer if you wish.
4. give that a LIGHT sanding with 800, wash and repeat the grease remover step. Use some spray putty to even it all out, sand, wash etc again.
5. give it a few light coats of colour, then some heavier ones, then some more. dont stay too long in one spot or come too close or you will get runs, dont do it too light or too far away or you will end up with sandpaper as paint. practice makes perfect here, i still make mistakes, lots of them. and they are annoying.
6. let it touch dry, then set it somewhere safe. let it cure for a WEEK! trust me, ive screwed up 2 paint jobs for this reason. ESPECIALLY if you are going to use a clear coat. For some reason when you spray the clear it just eats into the freshly dried paint, and BONZAI! your hard work is ruined.
if you cant find a matt clear, you might want to just keep a few cans of black paint handy. the clearcoat also evens up all the mismatched spray patterns you get in the raw colour coat with rattle cans.
hope it helps a bit, just some advice from a backyard DIY bum.