Mashed potatoes are normally made from boiled potatoes, not baked potatoes. You may need to add more liquid to get it to the desired consistency, but it still won't quite be the same.
As for how to make mashed potatoes: take your cooked potatoes. remove skins. MASH THEM. add about 25ml hot milk, 10ml butter, salt & pepper to taste. Measurements assume two medium sized potatoes, and can be adjusted to taste.