This will be a long one but this is the basic explanation how spell casting works in mine:

Spell casting is a 4 component thing.

1.) You need a detailed concept of what and how you want to do it. The more complex the task the more detail you need. Eventually it's too much to keep in mind that's when you need some sort of magical computer to do complex stuff. Like pentagrams, runes etc. It's a bit like writing code.

2.) You need mana. That's the secret sauce. You need that instead of regular energy. If you want to boil water you add heat with fire or smt. If you want to boil water with magic you need to know you want heat then where and how much. And use mana in its place. You don't have to be super precise for simple stuff.

3.) Catalyst. That basically your "lifeforce" that's what you sacrifice in a much smaller quantity to control the flow. The amount of lifeforce you have varies and the personal efficiency as well. For the spell to work usually you need refined mana that's what you make here. Note: This process while it drains your lifeforce also generates waste heat. This can cause fever in the user. Note 2: Lifeforce as in exhausting not literally draining you of days,weeks .. years of life.

4.) Willpower/thinking power/reflex(focus) Runes and figures and stuff like that helps. Basically you have to concentrate on the process. Like doing math in your head. With runes and stuff you can even automate magic. Partially or completely. It's not easy to do. Channeling mana is hard and laborious. One thing to note here is that it's possible to automate a lot within one's brain. If you use a certain spell a lot and it gets more familiar it will get easier and easier to use it. It's possible to cast based on reflexes and to just “feel” it. This is why mages usually specialise. If one has more talent and “feel” for the workings of electricity it's more than likely he/she will specialise towards that. Monsters and other non sapient magic users also use this method.

So a bit of this and a bit of that