Refer to the infographic, which shows how a concrete spur is used. (Courtesy of Allen Concrete)
Break out the concrete down the side of the rotten post where you want to fit the spur. A Hexagonal Point and Chisel Bar is ideal for this task.
The spur is concreted in place using a quick-setting post mix down the side of the post to be repaired, and the new spur. Then attach it to the broken post above the break using M10 coach screws; if you can access only one side of the post, or use coach bolts; if you can access both sides.