Well, I myself learned by searching the forums and talking to people - but my task was easier since I was porting from a WM6.1 factory ROM. Still, certainly WM2003 devices have got WM6.5 cooked ROMs on xda-devs so it is possible.
First thing you have to do is learn how to dump the ROM or extract a manufacturer's ROM update and dump that. If you can do the latter, go for it (easier). If you must do the former, you might want to learn a bit about your device hardware using pdadb.net and looking for tools used to dump from similar devices.
Taking the initiative and teaching yourself by reading the forums - so that your questions are targeted and specific - is helpful. Also being patient, not pushy, is a good strategy.