Typically you buy: Concentrated Nic (its cheaper) suspended in PG or VG, 100% PG, 100% VG, sweetener, (ethal maltyl and / or Sucralose) and Flavoring. There are other agents that smooth, or sour flavors but those are best left for later if ever.
The process is figuring how much you want to make total, what Nic level you want, flavoring amount, (15-25% is norm) and what PG/VG you want it all to be. Plug all the values into a calculator, or use shortcuts like Lox's, mix it up, and shake the hell out of it.
Most of the art (work) comes in figuring out what percentage of flavor you like, and what percentage you need to blend 2 or more flavors together. All flavors are not created equal. So a good tip, is to mix small flavor batches with no nic before you make a larger batch with nic and flavored to your liking.
Clear as mud right?