A Patronus is a kind of positive force, and for the wizard who can conjure one, it works something like a shield, with the Dementor feeding on it, rather than him. In order for it to work, you need to think of a memory. Not just any memory, a very happy memory, a very powerful memory… Allow it to fill you up… lose yourself in it… then speak the incantation “Expecto Patronum.”

(Source: billydarley)