I remember this video by a guy called Todd explaining how to punch nazis properly after Richard Spencer got his bell rung, reposted by a friend because Todd got suspended by the Twitter that was.
Should have saved it, back then, because it was quite instructive, and would still be current all these years later.
The gist of it, however, is as follows;
1) Make a fist with your thumb OUTSIDE, not inside, or you risk breaking your thumb. Fold it under, parallel to your knuckles.
2) Don't stop at the target, but punch as if trying to hit through that nazi face, like you're trying to hit something behind it.