It's not like a squat really. Make sure your form is good. It's about pushing your butt back, not about bending over if you get what I mean. The weight should be on your heels. In any case almost everyone with patellar tendinitis needs glute and hamstring exercises. And plyometrics too obviously, but be careful not to exaggerate.
Good luck with it, it's not an easy injury to deal with.