Cut two pieces off the board with your jigsaw. Make the length of one piece 10 inches. Make the length of the second piece 48 inches. Discard the remainder of the original board.
Cut a notch into the end of the 48 inch board. Make the notch 1 1/2 inches by 1 1/2 inches. Stand that board on its side. Drill two holes through the board, one 2 inches from the long end of the notched end and the other six inches from the same end.
Lay the 10 inch board flat on your work bench. Drill through the board in the same two spots as you did in the 48 inch board.
Lay the 48 inch board flat on your work bench. Slide the board so that its end is flush with the edge of the table. Have your assistant hold the board while you center the end of the board on the plywood base. Drive three of your 16d nails through the plywood and into the end of the board to secure the structure.
Stand the assembly on its plywood base. Slide a bolt through each hole in the upright. Slip the 10 inch board onto the bolts. Add a washer and nut to each bolt and twist both hand tight.
Stretch your tape measure from the top of your tillering tree. Make a series of marks on the face of the tree, under the lowest point of the 10 inch board. Space out the marks placing one every inch for 36 inches.