Place your heel on the bolster (such as one in our Knee Kit). You can be either lying or sitting; lying is a good starting place, but progress until you have done this successfully in a seated position. The goal is to completely relax your leg muscles to allow your knee to go as straight as possible. You may feel a pull in the back or even in the front of your knee. As with all of these knee stretches, that is okay, nothing bad is happening.
While you are stretching try to tighten or contract your thigh muscles and attempt to press your knee further down. This takes a fair amount of practice but everyone can and will be able to do it. Progress to hold the contraction for 3-5 seconds and repeat for 10 repetitions. On your last repetition hold the stretch for 30 seconds. If both knees are affected, repeat with other leg.
Aim for getting your new knee completely straight. Your other knee may have hyperextension, but total knee replacements seem to do better with just going completely straight, which is the goal.