Practice!
It's easy to put too much heat in...
Tomassini 's technique is interesting.
Some older body books describe hammer welding with a second person (or acting very fast) using a hammer and dolly while its hot. Tommassini's method seems better if you have the time. I haven't tried it. Almost all of the patch welds I've done are in locations with no easy way to hold a normal dolley in the back while hammering.
If you can MIG with less heat, then it should cause less distortion. I only have OA.