Here's a translation of a Woodsmith e-mail tip.
The jigsaw template guide consists of a quarter-inch thick plywood base sized to fit the base plate of a hand-held jigsaw.
Two runners or ledgers or edge grips are attached atop the edges of the base to grip the jigsaw base plate.
So the total width of the guide will be the width of the jig saws base plate plus the width of the runners. For example if the base plate is 3 inches wide and you use ¾ inch wide runners the total width of the plywood will be 4 and a half inches wide.
The plywood base is attached to the jigsaw base with double-faced tape. A three-eighths inch diameter hole is drilled in the forward center of the plywood base to accommodate the jigsaw blade. A second hole is drilled one-half inch in front of the hole for the blade. This second hole holds a guide pin made from a quarter-inch diameter dowel. The guide pin is three-eighths inch long and it protrudes from the bottom face of the plywood base. The guide pin is used to follow the edge of the template. To use the jigsaw guide, steer the jig saw with one hand and place your other hand on the plywood base to keep the guide pin in contact with the template. When you're finished cutting, you'll have a thin amount of waste to remove with a router--use a flush trim bit with the same template.