I’ll start with this… Squats are not the enemy. So you already know 1 way:
If you haven’t squatted since you were 2 then lets take it easy in the beginning.
1.) How to start squatting: Hold on to a post so you can maintain vertical shins. You don’t want your knees to pass over your ankles.
2.) As you become more flexible you can ditch the post.
Your goal is to be able to squat as low as you can and maintain balance.
Stretch Your Hips
Your knees can only move as well as your hips. If your hips are stiff then your knees will try and make up for the stiffness. Here are some stretches you can try: Pigeon, 90 90, Frog, Childs pose
This will help all of your joints. Trampolines are very low impact compared to running or jumping.
As you jump and land you get all the benefits of resistance training and your balance is challenged which will help with joint strength.
I prefer to call this soft tissue work. Basically, find a round, hardish object and roll all of the tissue up down and around the knee. This will help with blood flow.
Never Stop Moving
As you can see, all of the tips laid out here involve movement. It is absolutely true what we all say “if you don’t use it you lose it”. Our bodies are made to move so give it what it wants.
I hope this helps.