Start leaning the logic board back into the iMac.

Route the SATA data cable underneath the edge of the metal plate behind the logic board.

Continue routing the SATA cable up through the cable slot in the plastic frame.

Finish routing the SATA cable though the cutaway in the plastic frame near the optical drive, just below the GPU heat sink.

The SATA data cable should be laid over the SATA power cable that is already routed through this cutaway.