Originally Posted by ausdesign
As Auspiciousdna said and mark your quirks in all 4 corners. cut the bevel at one end of the top arc, hold it up in place and mark the length to the quirk line. fix it in place & then do the sides. - do the top mitre cut, hold it in place, see that the cut is closed nicely when you've lined up on your quirks & then mark the length and glue the joint & fix it at the joint & part way down the arc but only pin it towards the bottom. Do both sides & then fit the botoom arc and readjust the side arcs if necessary to get a closed fit on the mitre cut.
Its not unusual for mitre cuts to be off the 45 degrees. It depends on how good the window frame is installed.
A brad through the side of the arc to hold the mitre is good practice unless its mdf.