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Draw a dead center line on both T boards. Form the boards into a T, then place a piece of scrap wood the same width as the boards to support the top piece while building the jig.
Align the center lines drawn on the boards, then use a square or a combination square to make sure the top board is straight.
Remove the top piece and draw a line along the edge of the square (across the top of the bottom piece of wood).
Re-form the T, using the line that was drawn with the square to show the proper alignment for the top piece.
Use a brad nailer to put one brad in the pieces, then check to make sure the T is square. Adjust as needed.
When the jig is square, put in another brad to hold the pieces in place. Screw the pieces together for a more permanent hold. Drill pilot holes and countersink the screws.
Clamp a board tightly into the jig and route through the bottom piece of the jig and into the work piece. The next time you use it, the hole already cut into the jig allows you to align the board properly. The jig is bit and tool specific, so write down the bit and router used to make the routing guide right onto the jig.