this should be done by a physical or occupational therapist. Obviously, there is a reason why this person needs to use a walker. Some event or disease process has led to this point, and I would assume that this person has been in physical therapy for it.
Natalie is correct in what she is saying but his should not fall on your responsibility.
Since you would train this person only after a physician’s approval, I would look for parameters there and hopefully get a starting point from the physical therapist.
Once all that is established, you should first look at what the person needs to do in daily life, inquire into areas that are difficult and devise a strategy to help there first. Often just getting up from a chair is the first exercise. I always preferred to do exercises standing as long as you have a location where this is safe. Depending on the circumstances, you will have to settle for what is doable rather than what is ideal.