I tried several types of window frames before settling on this method. It is very slow but the result is rugged and handsome, and requires little additional trim.
First, a 4"+ by 8"+ slab is cut from a log with a chainsaw. I cut them freehand, but a chainsaw mill would work better.
Since my freehand cuts are pretty rough, I plane the slabs, and use a table saw to trim the edges (a jointer would work better, but it's still on my wish list). A dado blade is used to cut slots to accept a spline.
Then 2 slabs are glued together to make a true 4" x 16". Splines are simply slices of OSB.
The 4 x 16's are sanded.....
Dado slots are cut on what will become the exterior side of the window frame. The dado slots will allow mortar to key into the window frame.
Sharp corners have been rounded over with a router.
This is enough slabs for 3 windows.