First thing you need to do is measure the inside of the window reveal and add 5mm. The reveal length is measured from inside mitre point to inside mitre point. Transfer these measurements to your architrave timber.
Use the mitre box to cut a 45 degree angle on the end of one of the architrave boards. Measure along and cut an opposite 45 degree mitre on the other end of the architrave.
Place the bottom architrave on the wall. Place it 2.5mm down from the reveal. Check it is level. Use a dummy piece at the corner with a 45 degree angle to help position the architrave. Then nail it into place.
Repeat the process for the top architrave to mirror the bottom. Nail that into place using a dummy plate to check the position.
Measure the sides from long point to long point of the top and bottom architraves. It's important to measure both sides of the architraves because their lengths could vary.
Transfer your measurements and cut the side architraves to length. Remember to cut 45 degree angles at each end.
Slot one architrave between the top and bottom architraves to make sure it fits. Take the side architrave and add some PVA glue to the corners. Then slot the architrave in again and nail it into place.