How can i do simple physical therapy exercises at home to strengthen the knees?

Exercise: Depends if you are rehabbing an injury - if so see a physical therapist first to develop a home exercise program. A pt will show you the best exercises to recover and how to do them properly so you don't hurt yourself further.

Knee strength: Short arc and straight leg strengthening of quads and hams, rotate outward slr to work vmo, progress as tolerated to lunges & squats, but be careful.