You should rehab the injured knee area as follows:
1. Restore the range of motion. 2. Restore flexibility and strength of the injured knee area. 3. Regain balance lost because of the knee injury.
Stop any exercise that causes sharp pain. Continuing an exercise in pain will only set your rehab back.
Restoring the range of motion is critical because it lays the groundwork for future training. Range of motion should be restored in all 3 planes . If you had a knee injury, you might have to begin your exercises with partial range of motion (working towards full range) in one plane. You would gradually work towards including exercises that required sideways, rotating, twisting or turning actions.
Once range of motion is restored, you can then begin to introduce light weight training exercises and stretching exercises to strengthen the injured muscles, tendons and ligaments. It is also important to stretch and strengthen the areas around the injured area. Use of machine weights, isometric exercises (the injured area contracts but doesn't move), resistance bands or bodyweight exercises are all good in the rehab process.
Balance and proprioception (limb position sense in 3D space) must be restored or the injury will probably reoccur. When a part of the body is injured, the nerve cells in that area are also damaged. This affects your control and the stability of the joint structures. Once you have regained some strength and flexibility, you should begin to do balance exercises. You could start with simple exercises like walking in a straight line and progress to one-legged or closed-eyed exercises. You would then gradually progress to stability ball exercises, foam cushion exercises, BOSU exercises, etc.
The rehab process should not be rushed. Trying to come back too fast from a serious knee ACL injury will almost certainly lead to reinjury.