Now that the preparation work has been made, marking and cutting the individual lengths of architraves to size can be done.
For correct installation, care must be taken that all the marking and cutting is done accurately as making adjustments to a mitre can be very difficult unless you happen to be using a power mitre saw.
As for the sequences, I personally prefer to mark and cut the three components before fixing, which I find to be the easiest method as it leaves the door casing unrestricted for marking and testing.
The first piece to be marked and cut is the head. Begin by sawing an accurate 45 degree cut to one end of a length of architrave that would be when placed in position be long enough to span the width of the door casing and allow for the second cut and for any adjustments that may be required. Offer this piece up to the casing head with the 'Heel' of the 45 degree cut up to the point where the margin lines cross, ensuring that it is also running parallel with the margin guidelines along the head of the door casing. The drawing on the left shows the architrave in this position. With the architrave firmly held in this position, mark the position of the 'Heel' at the opposite end of the architrave at the exact point where the vertical and horizontal lines cross. This exact point is the mark for the second 45 degree cut.
Now that the head has been cut to length, the two legs can be marked and cut in the same way, but with the obvious difference that these pieces will only have one 45 degree cut at the top.
Begin by cutting one leg to a length that is long enough to accommodate the cut and place in position resting on the floor and up to the vertical guidelines (as shown in the drawing bottom right). Whilst held in this position, mark the point of the 'Heel' of the 45 degree cut where the guidelines cross, and cut it at this point at 45 degrees. The opposite leg is marked and cut in the same manner.
With all three pieces cut they can be fixed. Starting with the head, place it in position against the guidelines and with the 'heel' of the mitred corners up to the point where the guidelines cross at both ends. Drive a 38mm oval nail at the centre near the edge of the architrave, leaving the nail protruding. Fix another two nails about 30mm away from both ends, ensuring that the edge of the architrave is following the horizontal guidelines exactly, (leave the three nails protruding). Now place a vertical architrave in position so that it is parallel and up to the margin guidelines and both head and leg cuts are tight up to each other, and nail the leg about at the top about 30mm away from the top cut. Nail the bottom of the vertical architrave at the bottom, about 30 mm up from the floor, ensuring that it is still set to the margin at this point, doing likewise at just above the centre height point of the leg (above where the latch 'Keep' is located), and then at about 200mm distances apart from the top nail to the bottom of the leg. Do likewise with the opposite side.
To ensure that the mitres joints remain flush to each other, the two adjoining cuts should be nailed together by driving a 38mm oval nail down about 15mm from the top edge of the head into the leg.
All the nails can now be driven home, and punched slightly below the surface and the holes filled with the appropriate filler.