Grab potatos. Russet is my default. Clean the potatoes, about 1.5 pounds worth.
Chop into roughly equal chunks of about 1 cubic inch allowing for minor variation due to curved surfaces.
Cut in half length wise.
Cut each half in half length wise.
Cut into the 1³inch chunks.
Put in a two quart pot.
Add cold water to cover plus another two inches.
Add salt. Maybe 2 tsp worth.
Bring to a boil, reduce heat to medium high to prevent boilover when the starches release. Cook until fork tender.
Drain. Add 2-4 tablespoons of butter. Add 1-2 tsp of fresh ground black pepper.
Use a hand mixer to blend until everything is a consistent size.
Add 1/2 to 1 cups of milk or heavy cream. Or maybe half a cup of sour cream.
Blend until smooth with stiff peaks. Maybe add less dairy if you will be serving with some kind of sauce like pressure canned ketchup and thyme with pork chops or a gravy.
Add more salt if needed.

This is how I learned it as a kid. It's how I cook it today. But maybe you boil it on heavy cream and rosemary. Maybe you add roasted garlic. Maybe you add curry powder. Maybe you peel them (I never do).
Maybe you use a ricer instead of a hand mixer. But if you do make sure to peel them because skins and ricers don't mix.

The important thing is that there are no lumps, you use enough salt, you serve them the hot so the starches don't crystalize, and that you add enough fat to them. But absolutely, never any lumps.