There are many ways to do box jumps. The important thing when doing these is to land solidly on the box--your weight should be distributed over your whole foot, with both feet about shoulder width apart. You can technically just casually step down off the box, reset, then do the next solid box jump. This video shows a variation where you spring back up quickly after getting off the box, but no matter which way you do it, you should always land solidly on the box.