The most basic thing you can do with the shape mask templates from this site is use them as clipping masks, and add any paper to their shape. This tutorial is the same for clipping a photo to a photo mask.
What you need:
- a shape mask or photo mask
- any digital paper or your photo
Open your shape mask in the canvas you want to work in.
Add your paper to a layer above the shape mask.
Right click on the paper layer and choose "create clipping mask."
This will clip your paper to the layer below, which is your shape mask.