Thought this was going to be extremely difficult but once I started with these instructions it actually wasn’t that bad, though it took me a slow and careful 3.5 hours. Was quite nervous handling that power supply though!
I found the easiest way to connect the SATA cable was to not screw the plastic hard drive tray down until you plug the drive in. That way you can easily push the connector in and due to the cable you’ve got some wiggle room. You can position the tray then just lift one end of the drive up and secure the screw. Then just push the drive down.