Step 1 Set combination square with rule at 5mm. Place against inside of jamb and mark end of rule on jamb at corners and at various points along jamb.
Step 2 Using saw, cut architrave into lengths long enough for top and sides. Hold lengths in position against jamb so they meet marks. Transfer corner marks to lengths of architrave. Use these as short points of a 45° cut. As you cut each length, mark its position on the back.
Step 3 Glue and nail architrave to 1 side. (Use only a couple of nails at this stage.) Place top architrave so mitre joint is tight, apply glue and nail in place. Finish with other vertical architrave. If joins are not tight, adjust tops of verticals in or out.
Step 4 Finish nailing around architrave into both jamb and wall frame. To keep mitred joins tight, nail through side of architrave across mitre.