iFixit

Attach the other end of your new cable to the logic board where the optical drive was connected.

Carefully wrap up the extra cable and store it to the side of the hard drive.

Getting everything to fit with these particular cables is somewhat challenging. If you don't stow the cable correctly, it will prevent the case from going back on all the way, and could also damage the hard drives.

Before continuing, take a moment to double-check and make sure the SATA cable is plugged in securely to both the second hard drive and the optical drive connector.

Put the upper case back on!

You'll feel the tabs 'snick' to secure the case once you get it all the way down. If the tabs don't engage, check to make sure nothing is sticking out and blocking the case (you don't want to crush your new hard drive).