It can be a do-it-yourself project, provided one has basic carpentry skills and the right tools. The stool-and-apron design with mitered casing is simple and practical so, beginning with a miter saw and leading up to more is recommended. To make the trim sit flat, any bit of drywall at corner needs to be removed and shaving uneven hair off a miter may be required to make a tight joint.
Guide to measuring and making the trim:
To ascertain how much molding is needed, first measure the windows. Power miter saws are good for this work if one is adept at using them safely and setting up a large window fan on "exhaust" to draw out the dust is a convenience for working well. A worktable or a sawhorse is a good bet on which the cutting of a length of window stool that is 1-1/2 in. longer than the outside dimension of the installed casing can be done. Marking the center of the angled sill and the stool and aligning the two marks is required while keeping in mind that the stool horns should project equally out from the side of the casing as it does from the front. It is wise to wear safety glasses while doing so. Round over the ends of the horns and tack the stool to the rough sill with 8d finishing nails 10-in. on-center. After this, the cutting of the jamb extension can be done by ripping strips of 3/4-inch stock to the desired thickness for a full-length head-jamb extension, gluing and nailing it to ensure 1/16 to 1/8-inch of the head jamb is exposed. Head casing is marked with a 3/16-inch reveal on side and head jambs and then cutting a 45-degree miter on both ends of the head casing; tacking the casing to the jamb with 4d finishing nails and to the framing with 8d or 10d casing or finishing nails is to be done. By miter cutting each side casing and gluing miters, the joints can be locked with a final nail from top to side for side casing.
For a neat and detailed trim, a mitered return on ends of the apron would give a good finish. The scrap moldings can be used if short and glued and nailed to the apron to form a 90-degree corner at each end. On drying, the leftover can be scraped and the apron nailed to the wall framing and through the 6d finishing nails, slightly below the surface. If planning to paint the trim, then filling the holes with wood putty and sanding it smooth is recommended but if only wanting to stain and varnish, then this filling is done afterwards by filling nail holes using crayon-like putty sticks.

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