I guess since it's a kind of energy you harness it in as many different ways as you do any kind of energy. And same for its uses. Some are better at harnessing it in a way, others in another. There's Battle magic, sex magic, god magic, ink spells, wards, telekinesis, telepathy, teleporting, rituals, enchanted objects, summonings...
Usually to cast we use our hands, draw up patterns in the air, each pattern is a spell or part of a spell. It's like willing and pulling the energy into being what you want it to be/do. It's hard as hell to even get anything to manifest. There's also incantations in pretty much every language there is. It's vast, depends on who you are, what you have access to, what your culture is. Or whoever first made the spell, anyway.