Prise away the old skirting using a crowbar, place a block of wood between the crowbar and wall surface to prevent damage to the wall. Start from an external corner or where the skirting meets an architrave, but be careful not to damage the architrave, if this is not being replaced.

Measure the length of the skirting required. If the length of skirting is finishing on an external corner then mark the back edge of the skirting and use a mitre box to cut an angle of 45° making sure that the front edge is longer than the back.

If the length of skirting is finishing in an internal corner then rather than making a mitre joint, make a straight cut. Most mitre boxes have guides that allow for straight cuts or you can use a try square.

Place one length of skirting at 90° to the other, line up the edges and draw around the profile.

It is usually better to cut a profile of the skirting and butt them together. This prevents the mismatch that can occur when a skirting with a shaped mouldings is mitred.

Use a jigsaw or a coping saw to cut along the marked line, this will allow the skirting to fit neatly over the adjacent length.

Fasten the skirting in place, if the skirting is being fitted to a studded wall, a contact adhesive may be sufficient or you can nail into the battens. If the skirting is being fitted to a masonry wall then use the original fixing positions which are usually wooden wedges that have been inserted in the masonry. Punch any nail heads below the surface and fill the holes and any gaps between the mitred ends with wood filler, sand down and either paint or varnish.