Casting the Patronus

1. To successfully cast the spell, you must muster the happiest memory you
can think of (the happier the memory, the better the charm will work)

2. Begin drawing circles with your wand so as to increase the power of your spell.

3. You must then say the incantation, "Expecto Patronum"; the Patronus will come from the tip of the wand and can be directed towards a target by pointing your wand at said target.

Some witches
and wizards may manage an incorporeal Patronus, which resembles a mass
or wisp of silvery vapour or smoke. In some cases a witch or wizard may
choose to produce an incorporeal Patronus deliberately, if he or she
wishes to disguise the form it generally takes.
The incorporeal Patronus is not a true Patronus and while it will give
limited protection, it cannot provide the defensive power of the
corporeal Patronus, which has the form and substance of an animal.