All you will need to do is to pair up your defensive linemen. You may want to pair them off with a partner that is approximately the same size.
Have each pair stand facing you one behind the other. The player closest to you will be the "offensive lineman". The player behind him will be the "bull rusher". Have the bull rusher fit up to the back of the offensive lineman with his hat and hands. Have the offensive lineman lean back to give resistance to the bull rusher. On the first whistle, the bull rusher drives the o-lineman. On the second whistle, the o-lineman will sit down hard which will produce even more resistance for the bull rusher. Once the rusher feels this, he must now lift up the o-lineman and drive the hips through and push from the arms. Then, you can switch up the o-lineman and the bull rusher.
Make sure that the o-lineman is making the rusher work, if the rusher is moving the o-lineman too easily, then you must get on the o-lineman and tell him that he is not helping the rusher. Also, make sure that the rusher rolls the hips through when he feels the o-lineman sit down. You can progress this drill, by giving the o-lineman a silent signal (a fist in the air, a nod, etc.) when you want him to sit. This will allow you to see who is feeling the extra resistance and who is not. It will also tell you who is capable of bull rushing and who needs to get to the weight room. Always make sure that the bull rusher keeps his head up and never ducks it.
This drill is designed to teach the bull rusher to recognize one possible technique that an o-lineman may use to counter the bull rush and how to counter this counter.
Tell the players that the bull rush is just one pass rush technique. I would like to see a player use more than just the bull rush technique. They should work on the basic rip or swim as well, because they will not be able to bull rush everyone they play.