Here’s the process.
There’s magic energy in the air around you. It’s pretty useless. Then prometheus came along. He made something happen better than bread or butter… The magic gene. Now, your cells can store magic energy. Oh yeah! Now it’s NOT so useless!
Let’s see. The soul has a property that lets it manipulate magic energy, especially that nearby it. Let’s draw some energy in, shall we?
fwooooosh.
There we go. As you can see, we have as much magic energy as our body can physically hold. And it can travel pretty easily through our body because of the gene, and our soul manipulating it.
“But what element do we have, Mr. Percy? 
”
Well, that’s where minds come in. They’re tiny special places in your brain, that serve a special purpose. When you get old enough, you get your first magic, though how it’s determined is for another time.
Let’s say your first mind is fire. Well, guess what it does? Magic energy has special properties. If you shake it up like a bottle of pop the right way, it’ll turn into a specific magic. Your mind gives you just the right combination so that you know how to do this, but only for a specific magic.
Now, the first magic came from the soul. That was costly. But now, a great invention, almost as good as toasters and pop tarts. Magic circles.
If you didn’t know, whenever your soul manipulates magic energy, a little bit of it drips away. But what if we could use MUCH LESS magic energy in our spells and get an even better result? Introducing our Magic Circles.
They’re almost entirely 2-dimensional, and all we have to do is cram the design into our tiny, pea-sized brains. Then, it can effectively channel magic energy to make our spells for us, without having to manipulate magic energy as much. It also makes everything much more accurate and can form spells. 
Now, we can cast our spells. Just shake up a magic circle with little to no cost to our soul, let the magic energy channel through it and form into your magic with your cool channeling combination, and boom. Once your other minds are unlocked, you learn how to shake the energy up differently, and turn the energy into different magics.
Since the energy is in such small increments, if you twist it into your magic inside your body, it won’t hurt you unless you build it up. We use magic circles anyways now, and the transformation is done in there. After the spell is cast, eventually the energy will return to normal, and can be absorbed again.
How it turns into elements with certain motion combinations, nobody knows, but nobody cares lolololol. Thus ends my Tedtalk.
Nice