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.

The key behind this is to land solidly. You want your weight to be evenly distributed over your whole foot when you land, rather than, for example, landing on the balls of your feet.