From decks and porches to sheds and pergolas, a 6x6 vertical post is a common structural framing component. One of the more reliable ways to fasten boards to the post is by creating a notch.
To cut a notch in a 6x6, lay it on a couple of sturdy sawhorses and clamp it in place. At the end of the board, mark a notch that matches the size of the beam it will receive. On a handheld circular saw, set the blade depth to the thickness of the beam. Beginning at the end of the post, make a series of kerf cuts from one side of the board completely through to the other side. Space the cuts about 1/4 inch apart. Continue making kerf cuts until the blade reaches the marked bottom of the notch.
Leverage a hammer-claw or pry bar against the bottom of the notch and break the kerfed boards of the 6x6, snapping off the thin wood.
Use the hammer claw to remove excess wood, then use a chisel to clean the notch for a tight wood joint.
To double the beam, repeat the procedure on the opposite side of the post.
Connections can be made with carriage bolts, and the hardware can be concealed with wood plugs.