Measure the existing window sash with a tape measure. Make a note of the height, width and depth.
Measure the 2-by-6 to the depth and height measurement of the existing sash. Make a cut mark with a pencil. Cut the 2-by-6 to this size with the table saw. Repeat this step to cut a second piece of wood.
Measure the 2-by-6 to the depth and width measurement of the existing sash. Make a cut mark with the pencil and cut the board to this size. Repeat this step to cut a second piece of wood.
Cut a 45-degree, right-side cut on the two pieces cut to the width of the existing sash with the miter saw. Cut a 45-degree, left-side cut on the two pieces cut to the height.
Cut a 1/2-inch groove down the center of each piece of wood with the table saw. Attach the two pieces cut to the height to one piece cut to the width. Apply wood glue to the 45-degree cut corners and secure them with screws with the power drill.
Insert the window pane or piece of Plexiglas into the top of the frame and into the groove that you cut into the wood.
Apply wood glue to the 45-degree, cut corners and secure the remaining piece of wood with screws. Set the new sash in the window opening.
Attach the new sash to the top of the window opening with hinges.