Cut the plywood into a rectangle about 1/2" smaller than the size of your door opening.
- Use 1x ("one by") lumber to build a frame around each side of the plywood.
- You will be making a "sandwich" with the plywood in the middle.
- Make sure the joints of the frame on each side of the plywood are opposite. If joints line up, you will have a weakness.
Fill in the middle of the lumber frame with any kind of wood that looks nice.
Use "liquid nails" and short nails to attached the lumber to the plywood. Good luck!