I worked on this project a few years ago but wanted to post on it. It was pretty easy to do and didn't cost a lot. The main thing to keep in mind is most houses without existing window trim have rounded wall corners. These rounded edges created pockets that have to covered or filled when you lay the trim boards over them. Nothing major...just keep in mind.
Here was one of two of the window on the same wall of our master bedroom. "BEFORE"
Here is what it looked like after I nailed in the trim boards using a nail gun.
Starting from the bottom, here is what I used (all wood was common board from Home Depot):
- 1x3 window apron - Make sure to cut this to the width of the side trim (side casing).
- 1x6 window stool (shelf) - Make sure to cut this and place it before any of the other pieces. This will sit right up to the edge of your windowpane. Make sure you cut to fit with the bend of your existing wall. These can either the same width as your apron and side casings or a bit wider according to your preference. I would start wider just in case you decide later you like a shorter width you can always trim more length off.
- 1x3 side casing (sides of the window) - These will sit on top of the window stool and go to the top of the window where it meets up with fillet that runs along the top of the window. The inside edges of these will sit flush with the inside of the window (side jamb - see photos below) and not at the curve of the wall.
- 1x2 fillet - These will be the same width as the apron and side casing. It will be laying on its side so the narrowest part of the board will be flush with the wall (head jamb - see photos below).
- 1x6 top casing - You can play around with this width but my wife and I like the top casing to be a bit taller and more pronounced. Again this will be the same width as the side casing and fillet.
- Casing cap - I use multiple pieces of wood, each on their side for the casing cap. You can use one or many pieces depending on your desired effect. I used a 1x2, 1x3, and 1x4 with a 1/2 increase in width as they progressed towards the ceiling.
After I was done trimmI had to fill in the gap between the trim and the jamb. If I were to do it again I might try some drywall paper-faced corner bead and lay it down before I install the side casing and fillet. That way I wouldn't have to fill the gaps with as much material. This time I got some joint compound and drywall tools from home depot:
Please remember that drywall joint compound can very toxic to breathe in so please make sure you get some facemasks to wear while you are sanding it after it drys. Make sure that it is rated for joint compound sanding. The basic/cheap masks are not...
Here is what the gap looked like before the joint coumpound...
and after...make sure you allow the joint compound time to dry before you sand it especially if you are doing a think application like this. As it dries it could crack and you might have to apply it a few more times.
After it dries you can sand it (REMEMBER TO WEAR THE APPROPRIATELY RATED FACE MASK!!!) then use some caulk to fill any additional cracks, seams, or nail holes and then paint.
Here is the finished product! Let me know if you have any questions.