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According to the dimensions of your shower, use a circular saw to cut the piece of plywood into two equal pieces. This will be the front and back of the soffit.
Working in a large open area, such as a garage floor or driveway, lay down the piece of plywood and mark the center. Also mark the top height of the arch with the chalk line.
Keeping the chalk line base in the same spot, start at one end of the plywood, connect the marker to the end of the string and draw along from one side to the other, creating an arch line with the marker.
Once the arch is drawn, cut it out with a jigsaw. Repeat these steps to make one additional arch to complete the sides of the soffit.
Nail 2x4s down the base of the plywood, connecting the two together to form a frame.
Once the frame is complete, secure it to the ceiling of the shower surround, creating a curved soffit.