Wrap the haggis tightly in tin foil and place in a large saucepan of cold water. Simmer gently for 45 minutes per lb. but do not boil as this might burst the skin.
If you would rather use an oven; place in casserole dish with a little water. Wrap in tinfoil to ensure haggis is kept moist and heat at 180C (gas mark 6), until piping hot for approx. 1 hour . When ready to serve, remove from foil and drain off the excess water.
Haggis is suitable for microwave. Additional time should be allowed for larger haggis.
Source: Scottish Craft Butchers