Leaving skin on, cut potatoes into cubes
Add to a pan of salted water, enough to cover potatoes
Bring to a boil and simmer for 10 minutes or so until potatoes are tender (test with fork)
Drain potatoes, add butter to pan and put potatoes back in. Add milk and salt. The amount of butter and milk you add is up to you and how smooth or rustic you'd like the potatoes to be.
Using a fork start mashing and stirring until the potatoes are the consistency you prefer. I like them more rustic (lumpy).