Drill a hole, centered, approximately 2.5 inches in from each end of the plank.
Sand and dust off the plank, then wipe on a coat of urethane with a rag.
Sand the plank when dry; then, re-coat and repeat process.
Thread the end of rope into one hole, from the top, and bring it up through the other hole. The rope will run along the plank bottom like a giant stitch. Keep pulling and threading the rope on each end until the two lengths are long enough to tie onto the tree limb.
Loop the rope over the branch, and back underneath toward yourself. Pass the rope around the front of the stationary end, through the opening between them.
Wrap the rope end around the stationary part of the rope again and go under the limb, then pull the rope over the limb toward yourself. Make sure this pass of rope over the limb is parallel to the first pass.
Thread the end down along side of the stationary part of the rope, passing it through the loops you made when you wrapped it.
Pull the end through tightly, keeping the ropes parallel.
Repeat these steps for the end of the rope on the other side of the plank.