Check your miter at the second corner with your test pieces, and adjust if necessary to fit tight. Mark the opposite corner on the top casing, cut it and tack it up. Trim a window using the same ...

Nail the thick part of the trim to the framing with 2-in. brads. Check the Fit, then Cut to Length Cut a 45-degree miter on one end of the trim board. Adjust the miter as needed for a perfect fit.