I find it easy, get some pg and vg and mix that in a container to your preferred ratio. Then use that mix to add flavors and nic to. I bought a syringe at walgreens used to administer medicine to infants, it's calibrated to 10 mils. I add a little of my pg/vg, add the nic, then the flavor and top off with more of the pg/vg. Then squirt the whole thing into a 10 mil bottle and shake.
You have to be careful if you use 100mg nic, it's toxic and can be absorbed through the skin. I'm not brave enough to try the higher concentrations. But you can buy lower concentrations as well. I used a dropper to determine exactly how many drops made up one mil. So it's been easy to go from 10mg nic to 8, to 7, to 5mg.
I've just done plain flavors out of the bottle so far, but come Fri, or next week depending on USPS, I'm going to try and mix flavors.