There is no preparation if you buy peeled potatoes.
Cook the potatoes in water. Add quite some salt to the water. Cook for 15 minutes or so. Drain.
Put the milk on the heat. Add the butter. Take care, your kitchen will turn into a mess when the milk starts cooking without your attention.
Use a mixer or a masher to mash the potatoes. Take your time to mash it as finely as possible. You do not want any lumps. Puree sounds smooth, and it should be smooth. Keep an eye on the milk! Add the egg yolks, pepper and salt, and keep mashing.
When the butter has melted, and the milk started cooking, slowly pour it into the potatoes while you keep mashing. The amount of milk you want to add depends on what you want to do with it. If you want to create Puree di Papa you'll need less than when you want to eat it with Drunken Cow or Tender Tenderloin. Taste it, you probably need to add more salt.
Use the leftover egg whites to create meringues.