Darkstar sell their own mix, but it's fixed. Not that mixing 2 ingredients is difficult, it's just one thing less to do, and a lot less messy!
You need a syringe to measure, bottles, nicotine, PG and VG, plus whatever flavour you want. I just go with 50/50ish VG/PG, about 7 nicotine bottles thrown in, and done. No flavour for me. But i have used darkstar and various other sources flavours and they're all the much the same.
Mix up a large amount, then use that to fill up the little bottles on a daily or weekly basis. Saves making juice all the time. You can squirt a bit of flavour concentrate into the small bottles when you refill them from the big bottle.
Loads of vape juice calculators online that give measurements. Easy to do, and you can add more of anything to counter if something else is too strong.