First thing to do is to wash the shell in warm soapy water to get rid of any dirt, then dry it trying not to leave any dust or dirt behind.
Mask of the areas that are going to be the lightest colour first, so it is ready to paint the darkest colour. i always get some lighter fluid or motor spray at this point to get rid of and glue from masking tape and fingerprints left whilst masking off.
Next you need to get the paint ready, only use paint made for poly carbonate bodies. i like to use the tamiya spray cans as they fit nicely in the hand not being too big and they have a large range of colours.
Before i start spraying i run some hot water in the sink or something and put the spray can in to warm up the paint, this supposably makes it flow better. Now you are ready to spray, so in a well ventilated space spray a very thin layer of paint down first so it just looks like dust and let it dry totally. then slowly add thin layers until you think it is the right depth of colour. it is important not to rush as thick layers will cause it to flake off.
Once this colour is done you can peel off the masking and repeat the steps to paint the lighter colour/s over the top.
After the paint job is complete i leave the shell to dry completely for a day before cutting it out making sure all the edges are smoothed out using sand paper otherwise this can cause the paint to chip off around them areas.
hope this helps
rich