Peel the potatoes and cut into pieces of roughly equal size (A medium-sized potato is usually cut into about 6 pieces). Rinse the potatoes to remove any surface starch. Now place them in a large pan with sufficient salted water to cover the potatoes. Bring the water to the boil then turn down the heat and cover the pan so that it continues to boil gently.
Cooking time can vary depending upon the type of potato and the texture. Generally, however, somewhere between 15 - 20 minutes is sufficient. When you think that the potatoes are close to being cooked, pierce one or two of them with a fork. If the fork penetrates easily with little resistance, the potatoes are cooked. Remove the pan from the heat and drain off the water. Add the butter and, using a potato masher, begin to break up and mash the potatoes. Add milk (or cream if you want to be decadent) until the texture is soft and light.