I started by holding onto a stair bannister and squatting slooowly until I could actually get the full range of motion, then unsupported air squats, then I moved to an empty barbell. If you can't go straight to a barbell, or you have back/spine issues, belt squats are a great way to increase your workload as your capacity increases.

With those surgeries, starting under the supervision of a PT is a great idea, though. They'll know how to build you up safely.