Start by attaching the sight block to her bow and then install and set up the sight correctly (you may need to consult the directions—sight bar parallel to string, aperture in same place as string (also central plane of riser), etc.) and then take it (the sight, not the block) off of the bow.
If they know how to shoot off of the point and can do so reliably, have them start by doing that (at short distance 5-7 yards/meters). Then attach the sight and move the aperture so that it is near the top of the sight bar. Have them shoot some more arrows, off of the point, and ask them to see if at full draw they can see the aperture. Ask them where the aperture lines up on the target (visually). Move the aperture until, while shooting off of the point, they see it lined up with target center. Then ask them whether they could shoot just lining up the aperture with target center. They will almost always say “yes.” Have them shoot that way for a bit. Then ask them to move back substantially (from 3 to 5 yards/meters) and repeat what they were doing (the arrows will hit low). Ask them what they should do to adjust the aperture so the arrows are closer to the center. No matter what they suggest, have them do that! If they move the aperture the wrong way, the arrows will hit even lower. From such “experiments” they will get the idea and move the aperture the other way and they are will have figured it out on their own (albeit in a guided fashion). Then if they can do it once, they can do it again.
We don’t introduce string picture until later. Early on we emphasize good form, solid anchor, etc. If their anchor position is inconsistent, a string picture adjustment will not fix anything, simply make it more complicated. Many coaches are not aware that their strongest tool is the archer’s desire for the arrows to land in target center. If they can just get their archers to shoot, without trying an endless series of corrections, they will gravitate automatically to good form. Once their form is good, things like string picture can be added as checks instead of major tools.