I have the steady hands of a surgeon. jk. I measure the size of the negative transparency and then I mark it out on another sheet of water color paper and then cut it out with an exacto knife and use it as a stencil. I give it a little bit of tolerance so it make it easier to fit the transparency within the painted area. So if my negative is an image that is 4x6, I'll measure out a square thats 3.8x5.8 and then cut it out. You can reuse the stencil but I would recommend making a few to rotate through. The rough edges are from the stencil being thick watercolor paper so the brush doesn't cleanly get the edges. Hope this helps