Before you start, cut three lengths of casing — two for the uprights and one for the horizontal head section — to very roughly the length you need them. Cut them longer than they need to be, because the most accurate way to mark where to cut them is to hold roughly cut lengths in place and mark the cut location. Transferring measurements from jamb to casing is less accurate. The casing’s inner edge should be set back from the edge of the door jamb by 1/5" to 2/5" this is called the margin.

Step 1

Attach the First Upright

Mark off the margin position around the edge of the door jamb. Join the marks at the corners to provide a right-angled guide line (Image 1).

On the left-hand side, align some casing precisely with the margin. At the head, mark the piece where it meets the right-angled margin mark (Image 2).

Make a mitered cut, using the marked-off point to guide you (Image 3).

Reposition the casing, with its inner corner at the apex of the margin's right angle. Attach it with two or three nails at the top (Image 4).

Step 2

Cut the Head Section

Make a mitered cut, in the same way, on the head section, to join with the section already fitted on the jamb (Image 1). Check that it fits.

Hold the head section in position while marking off the cut for the other miter on the right-hand end of the length (Image 2). Cut the miter.

Step 3

Cut the Other Upright

Place the right-hand upright section of casing to mark off in relation to the margin requirement (Image 1), and cut this miter.

Nail the right-hand upright in place, securely at the top (Image 2).

Step 4

Attach the Casing

Check that the head length of casing fits, and nail it into position (Image 1).

Drive in nails along the entire lengths of casing (Image 2). Use six nails on the head casing and eight for each jamb.

Use a nail set to drive all nails below the surface (Image 3).

To keep mitered corners tight, you will need nails through both pieces. Prepare for this by drilling pilot holes (Image 4).

Drive a 1" finish nail into each pilot hole on the mitered corners (Image 5). It should run through the outer piece of casing and into the head section, to hold the joint tightly. Then fill nail holes and sand smooth, if necessary.