Which knee do I propose on?

The knee you use is not a legal or cultural requirement. The image of a proposal on one knee comes from old chivalric gestures, but nobody will check which side you used. The important thing is that you are comfortable and do not wobble or fall. If you have a knee injury or are wearing tight trousers, you can kneel on the other knee, or even stay standing if that feels more natural. Some people propose sitting down, or on a walk, or in the kitchen. The gesture matters more than the angle. If you do kneel, make sure you are on a surface that will not hurt, and keep the ring box secure in your pocket so it does not slip out. Practise the movement at home once or twice, especially if you are nervous. And remember, the moment is about the question and your partner's answer, not about performing a perfect pose. If you laugh or stumble, that is fine. It makes the memory more human.