Here's how I do it
Pick out my colours and sick a new no 11 blade on my scalpel.
Clean shell ready to apply about 4 coats of liquid mask
4 coats on, then wait for it to dry.
What I do when the mask is dry and I'm ready to put down my design is draw a centre line up the shell then I draw the design on one side.
Then I trace out the design from the first half onto some tracing paper the put the paper inside then trace it onto the shell. You don't really need to do this step you can just eye it up and make it match up.
Design complete now cut all the lines out with the scalpel, not too hard or you might go through the shell just hard enough to cut through the liquid mask.
Start your paint job off with the darkest colour first
I then painted the mirror chrome areas next because it needs backed with black
Put all your colours down from darkest to lightest and you are almost done (black, chrome with black backing, purple with silver backing, orange, yellow, white)
Put some decals on it and go race
Oople is a great place to get tips and tricks on painting and has a load of guys much better than myself, I'm sure they will give you good advice.
I would start off with a cheap airbrush and cheap oil free compressor I get my paint and other bits and bobs from Jon at RCS
http://www.rcsgraphicworx.co.uk/index.php