Once your Doctor gives the okay . . .
It's okay to start really, really slowly. I started at one minute a day. Next day, two minutes. Then three. Honest! The next week, four and five minutes. And so on.
At first, I did "exercises" in bed. Starting with head lifting on the way to something that slightly resembles a crunch. Leg lifts, arms. Before long I was excising out of bed. Oh boy!
Just start. Just start and do a little bit more today than you did yesterday.