Place an end of flexible molding on the doorway where you want the trim to begin.
Stretch the molding around the arched doorway. When you reach the point where you want the trim to end, make a pencil mark on the molding.
Remove the molding from the doorway. Cut through the pencil mark with a miter saw. Use a damp cloth to wipe off any sawdust from the molding.
Apply construction adhesive around the back of the molding.
Position the molding in place around the door. Firmly press on the molding to secure it in place. Use a damp cloth to wipe off any excess adhesive that seeps out from behind the molding.
Install 1-inch finishing nails into the edge of the molding and the wall. Set finishing nails every 6 inches around the remainder of the frame.
Countersink the finishing nails. Place a nail set over the heads of the nails. Hammer the end of the nail set to drive the nails into the molding. Apply wood putty over the holes left by the nails to create a smooth, finished look.