This section is from the book "The Boy Mechanic Vol. 2 1000 Things for Boys to Do", by Popular Mechanics Co.. Also available from Amazon: The Boy Mechanic, Vol2: 1000 Things for Boys to Do.
Procure a piece of heavy wire, one that is fairly stiff, says Camera Craft, and a pair of pliers and bend the wire with the pliers as shown in the illustration. After the loop is made to fit around the lens barrel the wire is bent at right angles at a point 6 in. below the circle. At a distance of 8 in. on the extending part of the wire it is bent as shown to form a clip for holding a sheet of cardboard.
The cardboard should be about 7 in. wide and of dark color, with one edge cut semicircular and notched. The size of the wire and the other measurements will depend upon the size and focal length of the lens. If a heavy vignetting card is required, it may be necessary to make the portion that encircles the lens double in length, bending it back upon itself to secure a firmer hold. This is a cheap and efficient vignetter that anyone can make in a few moments of spare time.