The drawing shows the tool I use to align the edge of a window apron with the casing above the sill. I take a scrap of 1x (in this case, a piece of baseboard stock) about 10 in. long. With a jigsaw, I cut a U-shape out of the middle of one edge. To align the apron, I press the tool against the jamb casing with the U going around the overhang of the sill. I slide the apron over until it meets the bottom of the tool, and then I drive a quick nail to hold it in position.
Billy McGraw, South Yarmouth, MA