method is easy.
find a saucepan that will allow a small, glass bowl to sit in it. add about 2″ of water to the saucepan.
the glass bowl should sit in the saucepan but allow a 2″-3″ gap between the bottom of the bowl and the top of the water.
place on stove under medium heat.
add leftover mashed potatoes to the bowl – remember, the water should not touch the bowl.
add 1 tablespoon of butter, per 1 cup of leftover potatoes you have in the bowl.
place lid on top. it will probably sit on top of the glass bowl, which is fine. allow to heat.
occasionally, gently stir the potatoes to allow heat to 