The tapering jig is held at the appropriate angle with a spreader bracket, which you probably noticed in the picture on the first page of these free woodworking plans.
To begin building this spreader bracket, cut a piece of 1x3 or 1x4 to 2" wide x 8" in length. Round over the edges with your jigsaw as shown in the image above.
Next, draw a pencil line down the long axis of the board at the center line, which should be one inch from each side. Then, using your cordless drill or power drill, make three 1/4" diameter holes, one at 1/2" in from each edge (on the center line) and the third at 1" in from one edge, also on the center line.