Measure the width of the windowsill between the side window casings.
Mark the measurement from Step 1 onto the 1-inch by 10-inch by 96-inch hardwood board, using a pencil.
Cut the hardwood board to the marked measurement, using a miter saw.
Sand the cut end of the board with fine-grit sandpaper in the same direction as the wood grain until the surface of the wood is smooth.
Apply a wood finish to the board, using a paintbrush. Allow the finish to dry for 48 hours before handling it.
Center the board in the window opening with one 1-inch by window-length edge against the outside edge of the windowsill.
Position one shelf bracket under the board 3 inches in from each end. Mark the mounting hole positions on the wall and the bottom of the board, using a pencil. Remove the board windowsill and the brackets from the window trim below the windowsill.
Hold the mounting brackets in place against the windowsill so the mounting holes line up with the marks on the window trim below the windowsill. Drive wood screws through the mounting holes into the window trim, using a screw gun.
Center the board on top of the brackets, and align the pencil marks with the mounting holes. Drive wood screws through the mounting holes in the brackets into the bottom of the board to fasten it in place.