How to Install an Outside Windowsill

Place the flat end -- the side opposite the curved end -- of a window stool up against the window's sill trim. Determine how far you want the stool to hang out to the left and to the right of your window, then mark the stool where you need to cut it. For example, say you start out with a 32-inch window and a 50-inch stool. If you want the stool to extend 3 inches on either side of the window, it should be a total of 38 inches, meaning you need to cut off 1 foot of the stool. Do this with a table saw.

Press the stool against the outside wall where you want it to go. Because it is wider than the window, it does not fit in place yet. Hold it in place by clamping it to the inside of the open window.

Place a pair of scribes inside the gap between the inside of the stool and the outside of the window. Use the scribes to determine the distance, then draw a line of this distance on the stool at the point where it meets the edge of the window frame. Draw a perpendicular line off of that line toward the end of the stool. Repeat this on the other side. The end result on each side is a thin rectangle drawn in the corner. Cut this rectangle out using a jigsaw.

Press the stool into place. If you measure with your scribes correctly, it should fit snugly into the recessed window frame with two "ears" extending along the outside wall.

Nail the stool into the bottom of the window frame with at least four 18-gauge nails. Use either a nail gun or a hammer to do so. The nails should pass through the top of the stool and down into the window frame.