Door architraves, or mouldings, cover the gaps between the jamb and walls and are often a decorative feature.
To DIY with an exact fit, use test pieces to check the angle of the mitresaw before cutting to length.
Secure the architrave, locking the corners with nails into the top and sides, recessing them below the surface with a nail punch.
Apply timber filler, allow it to dry, smooth with 180 grit abrasive paper, then paint.
You can buy architraves at hardware stores. The type of timber, profile and cost vary, starting at about $4 a metre.
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Position the first piece of side trim against the jamb. Use a combination square to mark a 4mm reveal around the edge of the jamb, then mark the mitre cut at the end of the side trim from the reveal line on the jamb head.
Cut the end of the side trim with a 45º mitre, then drill pilot holes and secure with 40 x 2mm nails. Cut and secure the top trim in the same way, measuring and marking from the side trim and reveal lines as you go.
Measure the exact length for the other side trim from the top corner of the top trim. Use a mitresaw to cut the side trim, drill pilot holes and secure with nails. Sand the trim, then apply primer and paint.