A few small lumps here and there are fine. But you do want to mash the potatoes until they are a soft and consistently smooth. Here's a quick tip to help avoid lumps. Cook the peeled russet potatoes in boiling water until they are very, very soft. Drain the potatoes using a colander over the sink and cover the potatoes with foil. Allow the potatoes to steam for about five minutes. The steam from the hot potatoes will help soften them. Place in a small bowl and mash with a fork or potato masher. These will become the fluffiest mashed potatoes ever.