All you need to do for this is blend all the ingredients together into a paste, pour it over your cut of lamb and leave it overnight. I use a large ziplock bag for marinading meat like this. Cook it as you like.
The photo is of a 1.5 kilo shoulder roast I did on the BBQ by the river yesterday (although a butterflied leg would have been better for that purpose), but I've used this marinade for all cuts and methods of cooking from oven roasted lamb legs to grilled fillets. It looks a bit burned in the photo because the coals were a little hot to start, but trust me when I tell you that it really was beautifully pink, tender and delicious. The sausage was a marjoram pork sausage tornade I secured with the leftover rosemary skewers after I'd used the leaves for the lamb marinade. It complemented the lamb extremely well.