Stain your wood to the color you choose.
Measure all four sides of the window. For example purposes we will say that your window is 24 by 24 inches square. You will take your measurement on the drywall from corner to corner on the same side.
Measure the width of your trim. For example purposes we will say it is 1 inch. You will add 1 inch to the top and bottom of your measurement for a total of 2 inches to make your trim 26 inches long. This is basing it on the example of a 24-inch window.
Put one end of the trim in your miter and cut it at 90 degrees. Make sure that you are paying attention to outward cuts. Do the same with the other end.
Hammer this piece on using your finishing nails and nail punch to slightly drive the nail below the surface of the wood. Make sure to match the ends of the trim up to the corners of the window.
Repeat these steps for each piece of trim. Each cut will be 90 degrees to match them up.