Measure the door frame opening
Cut the trim pieces to fit
Dry-fit the trim around the door
Mark the trim placement on the wall
Apply construction adhesive if needed
Nail the side trim pieces in place
Nail the top trim piece in place
Check that the trim is level and aligned
Fill nail holes with wood filler
Caulk gaps between trim and wall
Sand any rough spots after drying
Prime and paint or stain the trim