Cut the parts to finished dimensions.
Make a channel for the string. The 1/4" wide, 1/4" deep slots are located along the outside face of each end piece, beginning 1 1/4" from the top. Cut a matching string slot in one face of the tensioning stick. With a rattail file or sandpaper, round the slots’ sharp corners.
Mark and cut slots for the buck saw blade using a coping saw or scroll saw. For the shorter of the two end pieces, make a straight cut, 1 3/8" deep, starting from the piece’s bottom face. The slot for the longer member is 1" long and begins 6 3/8" from the bottom. To start this slot, drill a hole — just big enough to thread a coping or scroll saw blade through — at one end.
Cut mortises in the ends for the crossbar. These recesses should measure 5/16" wide, 1 3/4" long and at least 3/4" deep. Each mortise should begin 6 1/2" from the top of its respective member. To make the mortise, bore a line of overlapping holes with a 5/16" drill bit, and then square up the recess with a chisel.
Cut tenons on each end of the crossbar, 5/16" wide by 3/4" long. I cut these on a table saw equipped with a dado blade.
Using a table-mounted router, or sandpaper, radius the edges of the frame and tensioning stick. I used a 1/4" roundover bit on all long edges except those at the junction of the crossbar and ends.
Mark and drill for the blade-mounting hardware. First, drill a 1/4" diameter hole, 3/4" from the inside face and 1" up from the bottom on the short end piece. Next, bore a corresponding hole in the long end piece. It should be 7 inches up from the bottom and approximately 3/4" from the inside face.
Lightly sand all the pieces, remove dust and apply finish.
Assemble the saw and mount the blade, using one machine screw, two flat washers, one lock washer and one wing nut per end.
Wrap about six loops of chalk-reel line around the top, within the notches. Tie off the end. I like to rub beeswax on the string, which I feel makes it last longer.
Insert the stick between loops and twist until tight. The tension will hold it against the crossbar. The saw is ready to go. Reduce the tension when you are done working.
