Lots of good info here. It's not as difficult as it seems once you get started. It was kind of like learning to make coils (that was a big leap for me). Once you get the process down it's pretty easy and fun to customize your juice.
I started out with a bunch of syringes of assorted sizes, 16ga needles for PG flavorings and some 14ga for VG, a set of cylinders thinking I would use 1 for nic, 1 for PG and 1 for VG but often I just mix up test batches in the cylinders. I keep my PG and VG in 120ml bottles with a squeeze tip so measuring into the cylinder is easy and I can do fractions with a syringe if I need to. A small set of kitchen funnels is handy to transfer liquids to the 120ml bottles or when you're bottling up your nic for the freezer. I keep just enough nic out to use for a few months, the rest is in the freezer in amber bottles (I know there are some links here somewhere on places to buy them at a good price but it's way past my bedtime!)
Like others, my labels aren't fancy. I had a stack of file folder labels so I write the date, juice and anything relevant then, keeping the backing on it, I put a piece of tape over the label that's long enough to wrap around the bottle. It comes of easily when I finish the it. Some use a Brother P-Touch to make real labels and they look very nice. I haven't gotten into measuring by weight. It is more accurate, but there's also a reason I'm not a baker!
You can read about diacetyl and diketones. They're typically in buttery or custard type flavors. Some vapers avoid them, others don't worry. I'm in the middle. As I understand it there are more in cigarettes than ejuice so, to me, it's not a new exposure. If I can avoid them, I do, but I also use flavorings with them in moderation if its a key component to the flavor I want. Capella has "V2" and TFA has "DX" flavors that are reformulated versions without diacetyl. To me, not all of the new formulations are successful. I started with smaller sizes of the flavors to test and now have a few to order in bulk as I know they're in my regular arsenal.
A calculator is key. I keep all my notes there for substitutions or changes and tasting. I've started using this one:
http://www.e-cigarette-forum.com/forum/diy-e-liquid/480587-new-calculator-try.html. You can track your inventory, print labels, build a shopping list, search for recipes with flavors you use and a lot more.
Have fun!