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Step 8. Once you’ve made all the necessary modifications to the stool, slide it into place, and check the clearance between the stool and the window sash. The gap should be about 1/32 in., which is the thickness of a piece of cardboard on the back of a writing tablet.

Step 9. Shape the outside and end edges of the stool using a router. Usually this is done with a chamfering bit, like we show here. But a rounding-over bit or an ogee bit works too. Some people prefer a simple square edge that is rounded over slightly using 120-grit sandpaper wrapped around a wood sanding block.

Step 10. Use 8d finishing nails to attach the stool to the window jamb or sill. Before driving the nails, put a level on the stool to check for level and flatness. If necessary, shim the stool with cedar shingles.

Step 11. Once the stool is level and nailed to the sill, drill pilot holes in both horns, and nail them to the wall framing using 8d or 10d finishing nails.

Step 12. Hold a piece of leg casing on top of the stool and against the jamb. Mark a cut line at the top of the casing that is flush with the under side of the head jamb. Cut and nail the casing.