For Fingerling potatoes you usually need to just cut them in half. If they are a little larger you may want to quarter them. I also like using Yukon Gold and Red potatoes when making this. Just cut them bite size cubes.
Pile the cut up potatoes onto a rimmed baking sheet. Drizzle with 1 - 2 tablespoons of Olive Oil. Sprinkle with a little salt, and pepper to taste. Slightly chop about 1 tablespoon of fresh rosemary (or you can use dried, about 1 - 1 ½ teaspoons) and sprinkle over the top.
Using hands, lightly toss everything together, then spread out the potatoes in a single layer, trying to give a little room in between. Don't crowd them.
Bake in a 400 degree oven for about 30 - 40 minutes. Use a spatula to scrape up the potatoes and stir around halfway through cooking time. You want the potatoes fork tender and nice and golden brown.