A lump free, smooth, and not too stiff mashed potato is best. Extra lumpy or too stiff of a mashed potato will get stuck in the piping tip and be a nightmare to work with. If making from leftover mashed potatoes, you're looking for a consistency like buttercream frosting. Thick, but still spreadable. Add warm milk or half & half, a small amount at a time, until you reach the desired consistency. Stir in 1 beaten egg and you are now ready to pipe.