What they are talking about is the 50-pin JAE socket on the back of the drive, this gets adapted to a 40 pin 3.5" IDE drive socket. You then need a 44-pin 2.5" to 40 pin 3.5" cable which are easily obtained from Ebay.
For power to the drive, tap into the 5v and 12v supplies to the floppy drive.
You will also need a buffered IDE interface like the IDEFix 97 or EIDE 99 boards, these adapt the single IDE port on the A1200 into two ports.
Finally, remember that most laptop DVD and CD drives are set as Master so putting it onto a cable with a hard drive could cause the bus to hang as both will be set to Master.
Put the hard drive on one port, the DVD drive on the other and it should work out OK.
There, I hope that has explained it for you.
WTF