at my gym there are big tiles on the floor and there is a box made out of blue tape. you start on the bottom right side.. and you always want the line cutting you in half, so you have practice staying balanced when you're going up and down. when you're going side to side you want your lead foot to be touching the line. stay in your stance and with proper footwork, go up, left, down, and right. go around the box a few times. you can do it with punches too. just make sure to move first, then throw your punch(es)
and on the heavy bag, when it starts swinging around thats an opportunity to work on your footwork.
im glad you care about your footwork. it's very important, not only in moving, but in throwing punches.