Add thermal tape to cover the entire backside of the ESC. (Here's where you can use that excess material!)

Trim the borders for minimal excess.

Align the ESC with the Heatsink. Your cables should be sprouting from the enclosure side with NO CABLE PORTS. Your capacitors should not be near the openings of the enclosure.

Remove the thermal tape's backing and press down on the the corners to install in the same fashion you aligned it.

Once you install anything with thermal tape, it is difficult to adjust. Measure/align twice to ensure you only have to do it once.