Any answer that isn't exactly like mine should be ignored.
For god's sake, leave the skin on. It adds flavour, texture and probably the only nutrients a potato has. Make sure to boil them enough. It should be at least half an hour... when you stick the fork in them, the potato should basically fall apart from the stabbing.
Dump them in bowl and throw a bunch of butter in. Add green onion for flavour. Mash them around for a little while.
Put in a generous splash of milk. They should basically mash themselves after that. Literally, three more mashes after the milk and the whole thing is soft. If it is still chunky, you need more milk. The butter is more for flavour, the milk softens them up.
Obviously it is possible to have too much milk, so don't be stupid. Add small amounts in stages the first time you do it until you get the right consistency. And the milk doesn't have to be warm, as you aren't adding enough to cool the potatoes... ignore the Frenchie