Sketch out a design of your windmill. Most feature a small base section decorated at the top with four windmill paddles.
Build the base by nailing the two-by-fours together, making a square. Nail the plywood over the sides of the square.
Cut out four "paddles" for the windmill. Use a marker to draw the shape of the paddle on the plywood, then cut out with a jigsaw. The paddles should be approximately 2 feet long by 5 inches wide.
Screw the lag bolt into the top of the base structure. Drill a single hole at the base of each of the paddles, approximately 1 inch from the bottom.
Put a washer over the lag bolt, then place each of the paddles over the lag bolt. Spread the paddles so they are equidistant from one another, creating a fan pattern. Put a second washer on top of the paddles, followed by a nut. Screw tightly so the paddles are in place but rotate freely.
Spray paint the base structure and the paddles. Use bright colors. Consider freehand painting a couple of windows and a door. Set the structure over the miniature golf course hole.