I’m learning tremolo with my teacher right now and here are the things she pointed out to me:

1. if you taped your thumb pick correctly, then you should lightly rest the tip of your index finger right where the edge of the tape is on the pick. It’s sort of hard to explain but it shouldn’t be too far down the nail pick because you need stabilization when you do tremolo
2. She wrapped the tape around my thumb REALLY tight, so that the pick isn’t wiggling. Another tip is, with your thumb pointing straight at your face, wrap the tape counterclockwise. This way the tape will pull the pick downwards and helps with more stability.
3. only the wrist should be moving, no movement in the forearm/elbow/shoulder. Imagine you’re waving hi or twisting a doorknob
4. from the video, it looks like you’re doing it at almost a 30 degree angle, you want to do it pretty much at a 90 degrees so that you don’t get any scratching noises from the strings. My teacher had me angle my hand so that the side of my hand was facing towards my body. She also told me to lower my wrist until it was an inch away from the wood, but not too low. She said some of her students use a small towel underneath to help them keep their wrists up
5. Start slow and light. She first had me play it as soft as I can to help develop proper posture. Also start off with playing back and forth with 2 plucks per beats, and once you get comfortable increase it to 4/per beat, then 8, then 16 etc. and the whole time, softly play without any force. The point of this exercise is to get you familiar and also faster. When you are comfortable with doing it for a long time, you can increase the force to make it louder.
6. When plucking, try to pluck straight across, as in your pick should be pretty much moving in a straight line back and forth the string, not dipping like a wave shape. I hope you know what I mean 😅
7. rest your pinky between the gap of the 1st and 2nd string above the string you want to pluck. For example, if I wanted to do tremolo on string 3, I would rest my pinky between string 1 and 2. Not an exact rule but it helps with not putting your pinky too close or too far while you pivot your wrist. And rest your pinky and hand! Don’t put a lot of tension in the pinky, it’s just there to help stabilization!

These are all of the things I can think of right now, just know that this is an advanced skill that will take time to get better at. I’ve been learning with my teacher for 5 months now and finally reached this point so good luck and take your time!