Get an AT or ATX power supply to power the cd-rom + A1200.. The A500 brick isn't really up to it. Search this forum for info on how to use the ATX PSU.
What kind of hard drive do you have in the A1200 now? 2.5" or 3.5"?
The cable you make must take into account both devices you are going to install on the IDE bus..
It's possible to make a long 44pin IDE cable that has three connectors.
The first two are normal 44pin connectors and the last one is 40 pin. You only connect the first 40 wires to the CD-ROM drive's connector (the last one). The 44 pin cable's wires are thin. so be careful to make sure they make contact to the big connector (soldering might be an option if you have a proper fine tipped soldering iron).
