Just like beans. Rinse them and either soak them for a few hours before using, or simply cook them in boiling water like pasta. I have this lentil dish I make with brown lentils and butternut squash. I cook the lentils in vegetable broth (I just like the extra flavor) and water while the cubed up squash is roasting. Then as the lentils are finishing, I toss in the squash and whatever else. I only use enough water to cook the lentils (not enough that there's left over liquid), and use a lot of clove seasoning (among other things). It's divine.