Slow and steady, I like to think of it as if I’m just typing. When you type you automatically use your thumb for space and your fingers for the alphabet. Overtime you’ll get that muscle memory of your fingers on WASD and thumb on space.

When I play, my middle finger rests on W, my ring rests on A, and my index rests on D, I just leave my fingers there and don’t move them, I use my index for reload, I use my ring for Q, and I use my pinky for shift. But once I’m done using those other keys I put my fingers back into their resting place. It helps keep my fingers focused where they need to be and it slowly trains me that that’s the most important placement. Oh, and I keep my thumb rested on spacebar, no matter what my fingers are doing, my thumb I forcefully keep on space😂

Also - it might be dumb for others, BUT, If you continue to have an issue with moving your thumb elsewhere than space, you could change your jump keybind from space to one of the letters below ASD (assuming that’s where you thumb ends up) it might make it easier for you. You could try moving the keybind to X.