Choose a piece of wood. Look for a piece that has a nice wood grain pattern and no knots.
Cut the wood into two pieces. One piece should be 40 inches long and the other 51 inches. However, you can vary the lengths if you want.
Measure 15 inches from the top of the longer board. Cut 2 inches off each side of the board.
Mark 15½ inches from the bottom of the shorter board. Center the tapered end of the longer board above this mark. Trace around the narrow edge of the tapered board. Use a jigsaw to cut out the rectangle. This can be the hardest part of the whole project.
Slip the tapered board through the hole. Sit in your chair. Adjust the height and any other problems you see.
Mark the angles needed to make boards fit together neatly. Also, mark the floor angles. Use a saw to carefully cut the angles.
Sand, stain and finish your chair. Or, you can leave it in its natural state. Let the finish dry, and then you're ready to enjoy your new plank chair.