I recently did the IDE mod and went by the pinout
here. Don't bother with that "Dream IDE Reborn" thing; it's expensive for what it is and it's for a full size 40-pin IDE drive which still requires an external power source. 44-pin is the way to go as it's powered from the Dreamcast itself and uses the smaller 2.5" drives which you can actually fit in the Dreamcast itself. I put my drive where the GD-ROM would normally go. The IDE cable just fits to the right of the motherboard. You just need to cut a small section of the metal tray the motherboard sits on.
I also removed the laser mechanism from a non-working GD-ROM drive in order to make room for the HDD. You need the GD-ROM drive attached or the Dreamcast won't boot up. Apparently there's a workaround for this but I found this to be the easier solution.