Steam some potatoes. (Don't boil them. No one wants watery mash.)
Add far more butter than anyone would think is reasonable. (Like, 1/2lb of butter to 1lb of potatoes. Maybe more.)
Add salt and pepper. (No, more than that. You've under-seasoned them.)
Mash them until they're the consistency you want. (For really smooth mash use an electric hand mixer instead of a masher.)
To make them better still, put lots of wholegrain mustard or garlic in with the butter.