Just a llittle water in a closed pan, so it's slightly boiled and mostly steamed. Remember it's already cooked, so all you have to do is heat it up thoroughly.
Only ever cooked according to instructions which ,the ones I've purchased, is in a medium oven. The outer wrapping keeps them moist. Enjoy which ever way you do it
I think that it's more important to buy a brand that's actually really tasty (which some seem not to be) than the method of cooking.
I know that many will regard it as sacrilege but I'm rather fond of the canned haggis that Asda sells. (Grants?). It simply needs heating in a saucepan.
Otherwise I cut 'proper' haggis into chunks and zap it in the microwave.
I broke it up into a casserole dish, lid on, 1hr & 20 and decorated it with brown sauce on the plate. Yum yum! Something a bit "soggy" about microwaving I have found and I preferred it the oven way, although I'm not sure if I didn't displace the lid when I put the sweet potatoes in, as it had a slight crust on it, but this is what I liked about it.