The more you practice how to make a proper fist, the more natural it will become. To get your thumb to tuck across proper, you may need to actually use your other hand and fix the position/ stretch over your thumb when you practice, but it will eventually come naturally. The more "down" you try to point the tip of your thumb, the further you'll be able to reach it across your fingers. When i make a tight fist, I can get the bending joint to pass between my pointer and middle finger, and the inner aspect of my fist is completely flat. It comes from a lot of practice, however. Besides your thumb, you also need to focus on the alignment of your wrist. When you make contact with an object, a good technique is to have the impact land evenly across your first and second knuckles. An easy way to practice this is to do knuckle pushups on a hard wood floor. If you've never done it before, the pain will be pretty fucking intense at first. But your hands will callous quickly, and if you're doing it right, the redness will be, like I said, on the first two knuckle tops and a little bit of the finger space below that. By knuckle tops, I mean the ones which start where your hand ends/where a ring would sit if you were wearing rings on your pointer and middle fingers.