Stretching with knee injuries

Fighting pro mma and grappling for the best part of 10 years my knees have lost alot of flexibility but im still able to work the rubber guard to a decent level, just need to stretch more and more just worried about pushing it too hard, has anyone got experience with stretching whilst having knee injuries and im just on the look out for some good stretching routines, like i know all the basic stretches just for how long you hold each position and what stretches are best in what order.

Thanks for any replies people!

What type of knee problems do you have specifically?

Just both knees have been heel hooked to shit and the tendons feel under alot of stress particular if im in butterfly guard and the guy crushes my heel to my ass

do box squats as low as you can to help strengthen your legs through that range.

are the ones where your touch your ass offf your heels called hindu squats maybe? anyway do them using a chair for stability if you need too. that way your developing flexability and strenght through your injuted knees.