Measure the distance around the arch door using a measuring tape. Some trim goes as far down as the floor, while others go only to the wainscoting or the baseboard.
Place the flexible trim in the room where it is going to be installed and leave it alone for two or more days. This allows the trim to acclimatize to the humidity and temperatures of the room. This prevents expansion or shrinkage later on.
Cut the trim to the measurement you found in Step 1 using a handsaw. Depending on the size of the doorway, you may need one piece or more pieces.
Apply a thin coat of a clear epoxy adhesive to the back of the flexible trim.
Bend the trim and fit it to the arch doorway. A clear epoxy adhesive should have immediate grab, allowing you to attach the trim to the arch easily.
Secure the trim to the door further by putting finish nails through the trim every 1 or 2 feet.