20 weeks after ankle surgery & nwb, how to start walking? How much pressure may I put down if I don't have pain? How much can I push it? Pt in a week.

Can't answer. Sounds like it must have been a pretty big surgery if you cannot weight bear for 20 weeks. The only person who can answer these questions is the orthopedist who did the surgery. You would do best to call the surgeon.
Ankle surgery. You need to ask your surgeon these questions as only he knows the nature of your operation.
Gradually. Has your surgeon given you the okay on weight-bearing? If yes and you are simply waiting for pt to start, you should begin with partial weight-bearing using your crutches. You will have decreased propriception (sense of balance) and loss of strength so you will need to use crutches initially. Gradually progress to walking in athletic shoes and do calf raises. If you have any pain...Stop.
Talk to your surgeon. There are so many possible answers to this question and this is because of the vast number of procedures that can be done to the ankle. These range from traumatic to reconstructive to elective. These are questions that you should discuss with your surgeon and physical therapist.