My favorite way to use these pens is to draw my stitching design onto my fabric, and then to stitch over some areas adding texture and depth. See the photographs here for examples. Adding a bit of applique to some areas will make your project even more interesting, and you don't have to make the shapes exact as the outline of the ink takes care of that nicely. These are a fun free wheeling way to spend some time being creative.
The pens have a thin and thicker end. Color is permanent. Heat set with an iron if you like the extra assurance, but they seem to go through the washer and dry and remain bright even if you don't!
Great beginning point for children They can draw and then stitch or you can stitch their drawings!