It is fairly easy to create a wider window sill for potted plants. Choose a sunny window, preferably with a wooden sill, for the installation. Select a piece of clear pine to cut your new sill from. Rather than tear into a perfectly good window, install your extension directly on top of the existing window sill.
Measure the width of the window sill from inside to inside of the frame next to the bottom of the window. Measure the depth from the bottom of the window to the front edge of the window frame. You will be cutting a rectangular tongue on the back edge of the wooden sill as wide and deep as this space.
Choose a piece of pine with a smooth surface and as free from knots as possible. A 1-by-8 works well. Mark the board to length 8 inches wider than the inside of your window frame and cut it with a circular saw. Mark a rectangle, centred along one edge, the same size as the space you measured. This will be the back edge of the sill. Draw straight lines out from the inside corners of the rectangle to the ends of the board on either side. Cut away the rectangle spaces formed by these lines and the outside lines of the original rectangle from the back corners of the board with a jigsaw.
Rout the front edges of the board with a 1/4-inch roundover bit with a bearing. Set the bit so that the bearing runs along the edge of the board and the blade cuts a 1/4-inch round edge on the top of the board. Clamp the board to a sturdy surface with a C clamp. Run the router from left to right starting with the edge on the left corner, run across the long edge of the other board and down the edge on the right.
Sand the sill and set it in the window. Drill two pilot holes in the rectangle tongue about an inch in from either side and an inch from the window, Countersink the holes slightly to let the screw heads set flush.
Drive two 1 5/8-inch fine thread drywall screws through the pine sill into the wood of the window sill below to anchor it. Apply a coat of paint or stain to the sill to match the window trim. Use a fine bristle brush to apply the paint or stain. Allow it to dry for the label recommended time, then add two coats of clear polyurethane to seal the sill from moisture from the plants. Allow it to dry for a day before setting plants on your new extended sill.