Simple. I'm embarrassed it took me so long to try them!
Potatoes. The amount and kind you use is entirely up to you. I love baby reds for this, but I use russet, too. 3 large russet for our family of 4, and a bunch of baby reds.
I don't have measurements, because I don't measure. AND, everyone likes their taters different.
Boil the potatoes, until they slide easily off of a fork. Drain water, and put back on the stove to mash. You can turn your heat on low if you use gas, usually on my cooktop the burner is still warm so this isn't necessary. You want heat though, so you can get that extra water out. After they are mashed, turn the heat off, and add the butter and sour cream. I usually start with a tablespoon of each and go from there. You can also use: milk, cream, and cream cheese. I use whatever I happen to have on hand. Salt them, taters need salt! You can throw in some good herbs here too, and maybe some garlic. Play with your taters, don't be afraid to screw them up, it would take some talent!