It's really easy if you have the tools. You can "eyeball" it if you don't, but you'll only get it so close......

Install the rest, and check to see if the arrow is level. If it's level, set the bow on your foot with an arrow nocked. Look straight down the middle of the limb, "through" the arrow and onto the next limb. If the arrow is out of center, you'll be able to see it. Move the rest accordingly.