This story shows you how to trim out a window.
Before you start trimming a window, finish the surrounding walls. If you cut out a large opening for installing a new window, that probably means you need to install new drywall. Attach the drywall and apply three coats of joint compound, sanding between each coat.
If you will stain the new trim, paint the wall first. Apply stain and finish to the trim before installing it, so you will only have to touch up the cut ends and nail holes later. If you will paint the trim, you can paint the wall after the trim is installed.
The jambs must be flush with the wall surface. If the jambs do not extend out to the wall surface, install strips of wood to extend the jambs outward. If the jambs are proud of the wall, plane them down.
About 2 hours to install basic window trim; more time is required for more complicated situations.
Tape measure, hammer or power nailer, nail set, hand miter box with backsaw or power miter box, jigsaw, level, combination square, drill
Careful measuring and precise cutting
Install the window and the surrounding wall surface.
The casing and stool material of your choice; 3d, 6d, and 8d finish nails; wood putty
Use a combination square to draw the reveal on the edge of all the jambs. This is the width of the jamb edge -- usually 1/4 inch -- that will not be covered by the casing. Set the square and hold the pencil tip against it as you slide the square along the jamb.
To determine the length of the stool, hold a piece of casing flat on the wall, one side aligned with the reveal line. Make a light mark on the wall (top). Turn the casing on edge and make a mark one casing thickness away from the first mark (bottom). Do this on the other side as well. The distance between the two outer lines is the length of the stool.