The windows in the current house I am working on were set in a bit from the wall and I had a hard time finding window trim which would sit nicely over the corner so I decided to make my own from 2x4's. I cut the 3.5" side down to .75". Two cuts on the table saw hollowed out the corner edge of the board which gave me a nice rounded sill.
I sanded the trim down and finished it with linseed oil. I did the same with the door trim and routed out a beveled lintel, mostly because we got an awesome assortment of router bits that I was excited to play with!