She should put the arm nearest the target surface on the target surface and the other arm on the armrest of the chair. After the person has been transferred to the target surface, help her to stabilize. Keep her close to you so that you don't hurt your back. If you can afford it, a mechanical lift can save a lot of wear and tear on your back. Private insurance companies and Medicare will often cover all or part of the rental or purchase of one of these lifts.
Mechanical lifts come in a variety of models.
It's not as hard as it looks, and the manufacturer will provide you with detailed instructions, often including a video. The retail dealership should also provide you with expert help, including showroom or in-home demonstrations, and, of course, you can consult with a physical therapist or nurse for more help. Many people with weakness from neuromuscular disease can manage to wriggle onto a sling from a lying position, or at least can assist you as they move onto it. You'll need either a bed with siderails or a second person stationed on the other side of the bed.