Jimlepooh :
i7Baby :
Pull everything out.
Mask off what you don't wnat painted.
Prime before top coating. Use plastic primer for plastic. Sometimes pre wipedown is needed.
Topcoat it. Maybe add a 2pack clear to kepp the paint in good nic.
Let it dry rock hard.
remove the masking tape etc.
Refit the PC.
I don't really understand it @_@
For all the work you have to do to buy materials to paint a case and factoring in all the prep work needed... you might as well just buy a new white case for $130.
However, if you really want too... here is a How To Guide:
http://www.modders-inc.com/so-you-want-to-paint-your-computer-case-a-painting-guide/