Don't have someone hold her down, as that will make the experience extremely uncomfortable and unpleasant for her! Just work very slowly. Your puppy is only 4 months old right now, so you shouldn't expect her to be able to stay put for a really long time. Stand next to her while she stays, and reward her after each time, gradually building up the amount of time. You might also find it easier to have her do a longer stay in a down position instead of sitting, since at her age the attention span is still not as long. You should also start working on the "watch me" command, which will help with the stay. Point to your eye and say "watch", and reward her for looking you straight in the eye. Gradually build up the amount of time with the watch as well, so that she will stay focused on you and avoid distractions.