Over at pretty handy girl cristina from remodelando la casa has a thorough and detailed window casing tutorial to get your from start to finish how to remove a window sill install a new one and trim the window for a full makeover.
Framing around inside windows.
What you need measuring tape stud finder hammer nail set flat pry bar drill with screwdriver bit level combination square framing square reciprocating saw handsaw stapler.
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.
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.
There are several parts that make up the inside framing of a window.
Glue the head casing to the head jamb making sure its bottom edge lines up with the reveal line.