OK, you want "Do this", nothing else.
I assume you have one existing optical drive on one IDE port with a ribbon cable that has three connectors - one at each end, and one in the middle.
1. Check your existing optical drive's jumpers. Make sure they are set to the Master position according to the diagram on the drive itself. Plug into it the END connector of the cable.
2. Set the jumpers on the new combo drive to Slave (use its own jumper setting diagram) and mount it in the case.
3. Connect the middle connector of the ribbon cable, and the 4-pin Molex power supply plug.
4. Close up and boot. The new drive unit should be showing in My Computer, ready to use. If it came with a software CD, do what it tells you.