Framing out a window is such an easy and inexpensive project and adds so much character and charm to a space. I'm going to take you along step by step and explain how I framed out the window below.
Measure the depth of the window seal and cut out the notch using a jigsaw where this piece of wood will sit leaving 3/4" to sit on the front of the wall.
I then needed to cut a little bit off the back using my jigsaw because this window seal wasn't quite 7 1/2", it was about 6".
We then added a 1 x 2 underneath ledge, two 1 x 4 on the side and 1x6 across the top.
Pin nail everything in place starting with the ledge. You do not need a brad nailer because this project isn't structural. Pin nailing makes it so you don't have to fill in a ton of holes. The holes that a pin nailer leaves are so small that once you paint the holes will fill in with paint. Using 1" headless pin nails will ensure it goes through the trim and the wall, along with DAP trim sealant those pieces of wood won't budge.
Once everything is pin nailed you will want to fill in all the cracks with DAP molding and trim sealant. This sealant is great because it is ready to paint only after 30 minutes. DAP also has these awesome little tools that keep your hands clean and easier to clean up the excess sealant for a nice polished finish.