I have already said it:
“Safety is No. 1 priority.”
Let’s start with the correct fist. Your thumb should be in front of your fingers. Like this.
This way your fingers form a plain surface and your bones are protected.
The natural fist for some of you will be by entering your thumb inside of your fist or placing it upright next to it.
Both will leave your bones and thumb more vulnerable to fractures and the surface of your punch uneven.
By the way, clenching your fist is supposed to boost your memory, too. Not a bad side effect I’d say.