Measure around the rounded doorway using a measuring tape, including all the areas that you want to trim.
Cut a piece of flexible trim that matches the measurement you found in Step 1 using a hand saw. Flexible trim is made out of a type of resin that is easily cut.
Apply an S-shaped line of construction adhesive to the flat side of the flexible molding.
Press the adhesive side of the flexible molding to the edge of the doorway, bending the molding so that it fits closely.
Hold the molding to the door for a few seconds to ensure that it sticks. Construction adhesive has an instant grab and when you remove your hands, the molding will stay where you put it.
Stabilize the molding using finish nails and a hammer, placing one nail through the molding every foot or so.