The most effective way to thinly and evenly slice potatoes is with a mandolin. I avoided buying a mandolin for years because I was concerned about the safety of those exceptionally sharp blades, but they make some very cleverly designed units now that are very safe to use. I love my new upright mandolin with an adjustable, hidden blade that does not need to be removed to be changed or cleaned. Prior to my mandolin days, I often used the slicing blade on my food processor. This is a quick and easy method although the resulting slices are bit thinner than I prefer for scalloped potatoes. Lastly, you can use a sharp knife and steady hand to slice the potatoes by hand, if preferred.