Just do it in the air and make sure to do it in different directions: out to the side, in front of you or uppercuts!
To really tone: do a series of different arm exercises and be sure to ALWAYS keep your arms up! Don’t let them drop!
Another exercise is just to put your arms out to the sides (hands at the same height as your shoukders, arms straught) and circle them forwards and backwards (make circles with your hands) and without letting them stop moving on to boxing etc. It’s great to keep your abdominals tense during this, making sure one the arms are moving.