We chose a relatively wide (3-1/4 inches) beaded wood casing trim to go around our doors and windows. It's 11/16 inches thick. We used three components to create the base: a 1/2-inch x 3-1/4 inch.
Pry the casing trim from the jamb frame -- the interior walls of the window opening ... Check for level again, and adjust as needed with wood shims placed under the window frame. Secure the window in ...