1 If the arch is to be built above a doorway, start by removing the architrave. Prise it away from its fixings with a hammer and bolster

2 Then remove the frame and door stops. Lift these away carefully so that you do not damage the brickwork around the opening

3 To construct the template, first measure the distance across the top of the opening at what will be its widest point

4 Then transfer this distance to the bottom of a piece of plywood. Measure and find the midpoint of this line and mark it on the board

5 Fix a piece of string to the midpoint and tie a pencil to the top end. Use this to draw a semicircular arc across the board

6 Cut along the line using a padsaw, fretsaw or electric jig saw. For a through arch, mark and cut out two matching template sides

7 Check the template for size by holding each of the side pieces in position above the opening. Adjust them as necessary

8 If the arch is too low to allow sufficient headroom underneath, move it upwards to a new position and cut it to size

9 Prefabricate each side of the template separately. Use 25mm square softwood to construct frameworks across the insides

10 Once you have constructed both frames, nail the sides together using short pieces of timber cut to the correct length

11 Drill holes for plugs in each side of the soffit and screw the frame into position across the top of the opening

12 The underside of the template is made from a piece of bent plywood or hardboard. Wet the material to make it pliable

13 Mark and cut pieces of plasterboard to fit across the top of the template. Fix them in place with galvanized nails

14 Finish off the arch with two coats of plaster. Try to bring the top coat level with the wall on each side of the arch

15 The angle where the soffit meets the two outside walls can be formed and finished off neatly with the aid of a corner shaping tool