Cut down vertically along the length of the leg to the bone, then begin to cut around the bone. Scrape the meat away from the bone as closely as possible. Continue to work your knife around and lift the bone free. Spread out the lamb and trim evenly.
Lots of Great Video out there. Jamie Oliver here.
My wife and I have used this recipe a number of times, and have found that the marinade, in and of itself, is excellent for any type of lamb. This also makes wonderful sandwiches if there are any leftovers. :