Measure around the edge of the wood stretcher every 12 inches with the measuring tape, and make a mark with the permanent marker. Include a mark in each of the four corners. These will be guide marks for pre-drilling the holes.
Repeat this process of measuring and marking on the Plexiglass so that when the Plexiglass is laid on the stretcher, all the marks line up.
Drill holes where the marks are on the wood stretcher with the twist drill bit in the electric drill.
Put on the respirator. Drill holes through the Plexiglass in the same manner.
Line up the Plexiglass on top of the wood stretcher so all the holes are in the same place. Put a washer on the screw, and screw it into the hole. Repeat for each hole.