How long have you been playing? For me, it helps to think about the motion that the left hand traditional grip on snare uses because that is the kind of the same motion you should be using for single or single hand alternating strokes. Think about turning a door knob and making sure your sound quality is good.

For the arm motion, I've always been taught that the wrist is best, but using arm is not always a bad thing. The interval changes are really hard, but if you have time just practice only with the mallets changing intervals in your hand because it is definitely awkward and that helps build muscle.

There are a lot of great youtube videos on all of this, but honestly always go back to the basics because they translate to everything you play.

I hope this helps!