1.

start off in this pose, then move the right arm up and the left arm down.
2.




3.
now start moving the arms in the opposite direction they were going








4.
once your in the pose you started off with, but flipped horizontally, make 2 more frames making the arms move opposite once again.
Finished Product:

I hope this helped, and rep if used
something extra:
animation in full body form.
