How can I build up knee strength after knee surgery?

I recently had arthroscopy knee surgery, I had a medial meniscal tear, and a lateral meniscal tear.

I have been doing physical therapy but I want to do more as I feel my routine is lacking. What can I do??? I had surgery on 6/8, it's now 8/15 and I am still in pain when using stairs. Any help is appreciated.
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