measure 8 times, measure again, double check that measurement, and then cut.
get the right tools, because it makes it nice and easy.
check out the stores you wouldnt think have stuff for computers, and keep your mind open. sometimes you find some pretty cool modding materials in the most random places.
take your time, be safe, and dont get all mad if you make a mistake, just try to work around it...
and above all else, have fun, because if you arent having fun modding your case, then maybe you shouldnt be modding your case?