I removed the platform, drilled through the lip, then about 1/2 through the base of the transom mount. I had to drill into the base of the transom mount to enable me to properly thread the lip - other wise the tap will bottom out on the base and not cut the threads deep enough to allow you to tighten the bolts.

I did not drill the platform part of the bracket.

The way this works is by putting pressure (IE: filling the void) between the transom bracket lip, and the swim platform bracket, essentially pushing the platform part of the bracket tight against the transom bracket.