It depends on your state of your knees. You can choose where to start from the next ordered list: easy massage of the knee pads, pushing inwards, up, down, left, right, then while standing lift one leg up (bent) and circle with your foot one direction and to the other (10 times each, you may hold on your chair or something not to fell off), then whiles standing both legs on the ground, bend your knees a little and start circling both of them in the same direction, then the other (10 times), then also while standing, slightly bent knees, circling them outwards, then inwards, then you may do 10 air squats, and if you really strong, you can do additional one leg squat for both legs.
Actually I'm not a specialist in sports but I'm following an app for sports, also you can find what you want on YouTube I guess.
I don’t know many specific warm ups that target the knees, but generally what works for me is plies (bending of the legs in a turned out position) work for an over all leg warm up. Hope that helps