With a small, very sharp paring or boning knife; scrape away the meat attached to the bone.

Start at the fat that surrounds the chop and scrape upwards to the top of the bone.

Finish it off by grasping the bone with a cloth covered hand ‐ a dish towel works ‐ and pull upwards.

Alternatively, this video from the CIA uses a different method:

How to French a Rack of Lamb (Step-by-Step) by Chef Avi