Measure out the exact cut that you ll need for around the window cut that section out so it almost looks like a big hat and then lay it right down on the windows ledge like so.
Framing around inside windows.
Glue the head casing to the head jamb making sure its bottom edge lines up with the reveal line.
The casing is the framing that goes around the top and side edges of the window.
With a miter saw make square crosscuts at those marks.
There are several parts that make up the inside framing of a window.
The style and material used to make the casing.
Hold a length of casing across the top of the window and mark where it meets the inside edge of each side casing.