To slice your mushrooms: Break off the stems and slice the tops into 3 slices. (I say this because I pick the big tubby mushrooms).
Slice butter into several pieces.
In a 10 inch skillet:.
Melt the butter in your skillet over medium heat.
Place all of your mushroom bits into the skillet, including the stems.
Stir well, coating the mushrooms with the butter.
Turn your heat to medium low and cover your skillet.
Check and stir every 10 minutes or so, so your mushrooms continue to be basted in the butter. They will form their own liquid, this is fine.
When your mushrooms are dark and soft they are ready, at least 1/2 hour. If you need to you can turn the heat LOW and keep them until the rest of your meal is finish. Once you've turned the heat down you needn't stir as often.
Serve on the side of your main meal as a compliment.