To complete the assembly of the tapering jig, attach the spreader bracket to the jig as follows:
Position the slot of the spreader bracket over the left carriage bolt and the hole over the right carriage bolt. Tap the bracket down into place. Then, add a flat washer and lock bolt onto the right carriage bolt and tighten until the bracket is snug, but not so tight that it won't rotate on the bolt.
You should now be able to move the left bolt through the slot, widening and narrowing the tapering jig. If satisfied with the freedom of movement of the jig, attach a flat washer, followed by a lock washer and then a wing nut onto the left carriage bolt.