This section is from the book "Complete Self-Instructing Library Of Practical Photography", by J. B. Schriever. Also available from Amazon: Complete Self-Instructing Library Of Practical Photography.
Making Oval Masks Of Various Sizes Without A Trimmer. Begin with the making of the first cut-out from one of your regular forms; then, for the next tint, should an inch margin be wanted, make the trimming guide as follows:
1178. Take a piece of stiff post office paper % of an inch wide by about 2 inches long, slightly tapered at one end. (See Figure No. 8.) Bend this paper double at the dotted lines, or just a trifle inside of where the strip begins to taper. To make a mask with an opening one inch larger than any mask you may have, measure one inch from the bended edge and with a sharp pencil pierce a hole in the guide.
Using The Guide. Lay the paper oval cut-out on a sheet of glass. Hook the guide, tapering side down, underneath the cut-out. Place the point of a sharp pencil in the little hole and move the pencil, which will draw the guide around the opening and give an accurate oval line exactly one inch larger than the first cut-out. (See Fig. 9). Then, with sharp shears, follow the line carefully and the mask is made. A desired size can be made by simply piercing holes in the guide according to the size margin you may require. If a very wide margin is wanted, it is best to have the under flap a little longer, from a quarter to a half-inch of where the pencil will operate. If the lower flap is too large it will interfere with the movement of the pencil. Always use glass under the cut-out, in place of a card, as the guide will move around and follow the opening more freely.