The best way to do it is to use the cloth simulation system, if you have it in your C4D package. It's pretty simple. Position a subdivided plane over your cube, convert it to polygons, give it a Simulation>Cloth tag. Add a cloth collider tag to the cube. Press play. Adjust the cloth tag settings to taste.
If you don't have cloth you can get a so-so result using a soft body simulation tag.