The Scarffer is a unique tool designed by boatbuilders for cutting accurate scarf joints in plywood up to 3/8″ thick. It attaches easily to most circular saws and is easily removed. The saw must have a base plate extension (outside of the blade) ½” or wider
Here are the use instructions by West System: https://www.westsystem.com/app/uploads/2022/12/875-Scarffer-Instruction.pdf