There are very few ways you can improve extension. By far the best tip is to ensure you are walking with the correct gait which is 'heel-toe' and stride and taking a decent step length, not shuffling because you are nervous of tripping. Don't obsess over this, try to make your stride as natural as possible. Extension is often more difficult to achieve than flexion and often lags behind but the most important thing to remember is that extension is most often blocked by swelling (=pain) which is best dealt with by managing the swelling and pain. If you attempt these exercises, it's quite possible you will actually increase the swelling and make your extension worse. The only thing I would suggest is to rest your heel on a stool so the knee is unsupported but do ensure the stool is not too far away from your knee. Mid calf as in this image is about as much as you should use. NEVER just have your heel on the stool and NEVER apply weights or press on the knee. It's actually better not to do any of the numerous exercises you find about the internet as they can make a bad situation worse, much worse.