Do you thin out the bottom layer of hair to make the top layers poof?

To thin your hair, cut it with thinning shears.

However, if you want to make your hair "puffy" you probably shouldn't thin the bottom layers. Try "teasing" them to make the bottom layers super puffy. Take a comb, and pull your hair tight. Then brush your hair in the opposite direction. You can also try curling it, or getting a perm to get more volume.