iFixit

Route the new SATA cable through the indentation that the primary SATA cable goes through and into the optical drive bay.

Set the logic board back into place, being careful not to catch, cover, or pinch any cables.

Be sure that the SATA cable has as much slack as possible and that it is not tangled with any other cables.

To secure your new drive in your iMac, you will need to place high strength double-sided tape at the four corners on the side of the drive with the SATA connectors.

Peel the backing off one side of the double sided tape.

Press the exposed adhesive side of the double sided tape to the corner of the SSD.

Be sure not to cover the SATA connectors with the double sided tape.

Repeat the above process for the remaining 3 corners.

Connect both the SATA data and SATA power connectors to their respective sockets on the SSD.

Peel the backing off of the double sided tape.

Carefully set the SSD into the iMac optical bay as close to the lower left corner as possible.

Do not to repeatedly stick and un-stick the tape, as the adhesive will become weaker.

Once you are happy with the SSD's placement, firmly press down along its edges to ensure full adhesion.

When re-installing the optical drive, the SATA cable will be sandwiched between the foam padding and the optical drive.