Window Finish Work
When dry-walling, paneling or whatnot, come to the edge of the rough opening. Cut the stool from a 1X4 (or greater depending on how far you want the stool to protrude beyond the apron). It should be notched out so that it fits snugly against the window and wraps around to extend about one inch beyond where you want your casing leg to stand next to the opening. If necessary, cut thin pieces of plywood to be used as spacers beneath it so that it will fit snugly to the bottom of the window frame. Install spacers and stool by predrilling holes and using 2 inch finishing nails as fasteners.
Now, measure the width of the rough opening at the top. Cut a 1X4 or similar dimension lumber to that length. Measure the width this board must be in order to fit against the window and flush with the outer edge of the drywall. Rip the board to the proper width using a table saw. Install the board into the header using finishing nails, here again you may need spacers. Do the same thing for the jambs on the both sides of the opening, only this time, you will measure from the top board to the stool.
Cut the casing legs to length (from the stool to about 1/8 inch past the lower edge of the top board). Realizing that you want to “reveal” 1/8 to 1/4 inch of the jamb, nail your casing legs into place. Cut the head casing so that it will extend about 1/8 inch beyond the sides of the casing legs. Again nail into place through the drywall and into the rough frame. Finally, cut the apron to a length equal to the head casing. Install beneath the stool also nailing into the rough frame.