Use 1x2s to trim out the window. Start with the sides, 1x2s cut at 21″. Then measure, and add the 1x2s to the top and bottom, keeping the bottom trim flush with the sides. The top trim piece will overextend 3/4″ on either end. The measurements given are to guide you and may not be exact with your projects measurements. Once the window has been trimmed out, add the sill, a 1×3, keeping the back edge flush. Remember to trim out both windows. The measurements are the same. Use 1 1/4″ nails and glue to nail the trim to the plywood.