Mousse can be used as a curl booster for normal to dense hair (so before 'Gels, Custards and Spray Gels') or as a final product to lock in the curls on finer hair types after you've applied your 'Curl Booster' product. Some people remove water with a 'Flour sack towel' (available on the site) before adding Mousse if their hair is baby fine/thin. You can also use mousses for the 'Smasters Technique' (search on YouTube) and it works wonderfully. You can also apply Mousse right into your roots on wet hair before you dry it for more volume, and some clients have been known to apply it into the roots on dry hair with great success.
Make sure to apply it when it's still foamy (don't squish out the bubbles) and do a good job of scrunching it in until all bubbles are gone.
Read the product descriptions for more info on how to use each one.