Place the flexible trim you are going to use for the window in that room for 24 to 48 hours. This allows the trim to expand or contract in accordance with the room’s humidity.
Wash the area where the trim will be applied using soap and water, and allow it to dry completely.
Measure the distance around the curve of the window you want to cover with trim using a measuring tape.
Cut a piece of flexible trim that matches the measurement you made in step 3 using a saw. If the trim is narrow, you may be able to use a utility knife to make the cut.
Spread a thin layer of wood glue along the back of the flexible trim.
Bend the flexible molding and fit it to the arch. You should be able to bend it easily in your hands.
Press the flexible trim to the arch, and hold it there for several seconds as the glue takes hold.
Allow the glue to cure completely. Wood glue’s cure time depends on the manufacturer, so heed the directions on the packaging.