You just design it as it's own layer within photoshop. White on a transparent background would work best. Save it somewhere.. When you are finished working with a photo, add a layer to it, then paste in the work that you previously made. You will probably need to make the new layer like 75% transparency or something similar.
You could also set it up as a custom brush in PS. Just as easy w/o the need to open a whole 'nother document to use as that layer. Either method would be just as effective.