If you aren't using extension jambs, you place a piece of sill temporarily where it will go to serve as a reference for the ends of the side casings. Install the side casings, then carefully nail up from the sill to the ends of the side casings, pre-drill for safety. You can clamp and glue the sill in the groove provided if you want to. Standard pieces of window sill will not have the tongue to fit in the Andersen groove. Some folks will cut the tongue, others will just fit to the window with a butt joint. It depends on the material, some are beveled underneath. With this type, you cut a rabbet rather than a tongue. After the sill is nailed up to the side casings, the apron goes up tight against the sill. No nails are used from the sill into the apron. An alternative is to place the apron then nail down into it where the side casings will cover the nails.