You can make it as a meatloaf, since that's what it is, but to make real haggis, you need the help of a friendly butcher who will "gift" you the necessary lamb-innards.

Using a food processor, chop half of the lamb, the liver, onion, suet, egg, salt, pepper, sugar, ginger, cloves, nutmeg and water until finely combined. Add all remaining ingredients, mixing thoroughly until everything is just mixed together well. You want a matrix of larger and smaller pieces.

Easy: Put the mixture into the greased loaf pan and flatten the top. Bake 45-55 minutes in a pre-heated oven (350°F / 180°C) The centre of the loaf should be firm when pressed. Leave in the tin for around 2-3 minutes to cool. Gently turn on to a serving plate and serve right away.

Authentic: Beef casings come packed in salt. Thoroughly rinse inside and out, and tie off one end. Fill with the mixture, taking care to not stretch it more than about 6" wide, because it might burst when boiling. Tie off the other end, cover with water in a pot, and simmer for 90 min, then serve in the authentic way, which involves marching it around the dining table preceded by a bagpiper. How much stuffing is too much? A casing burst on me when I stuffed it to where it was more than 6" wide.

Serve with champit tatties and bashed neeps, of course.
TIPS:
Crucial instruction: take a pin and prick it all over about twenty times before boiling. Otherwise a messy time is guaranteed for all.