Remove any extra plaster material so that template can sit closer against the curved edge.

Establish where the curve starts on both sides of the reveal. Mark with a pencil on the opening.

Do another mark 100mm below on both sides.

Fix a nail in at this point with a portion of the nail exposed to support the template material.

Measure the size of the template material required allowing excess material outside the arch area

Locate a piece of a semi-rigid material such as thin plywood or MDF, 3-9 mm thickness is suitable.

Cut the template material to the required size.

Place the template on top of the two nails and tape into position.

You may have to secure the template in place with extra fixings.

Trace around the curved edge on the inside face of the template with a sharp pencil.

Pull out the fixings and the tape and remove the template.

Label the template including the location of the window arch and what face the line is drawn on. This demonstration represents the back face of the architrave.