You could do this much better with a simple bowline. No need for the overhand knot or the movable tautline hitch, just make a bowline with the loop part through your swing.
Tie the overhand knot first, then feed through the board. You want the rope in the tree first, this makes that possible. Makes it much easier to level the board if you’re using two separate ropes.
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You repeat the process on the other side! The ropes are separate from one another! For the entire tutorial, visit here fernwayhome.com/?p=531. Hope this helps!