It's all been covered, but I'll post anyway...
For test batches, first I mix up 30-60 ml of everything but flavor (i.e. PG, VG, nic, water or vodka, and a short amount of sweetener). I make this base a couple mg/ml higher than my target, but I'm not really concerned with accuracy of mg/ml at this stage. I then use a syringe to divide that out into 3 ml aliquots in 5 ml glass vials. I then add different flavor combinations to those aliquots. Sometimes, if I have a good base flavor I like, I will mix up a big batch of that, divide it up similarly and test the small aliquots with different enhancer flavors, different amount of sweetener, etc...
Once I have a recipe I like, I scale it up to about 15 ml to make sure I like it enough to vape more than a few ml of it. If the 15 ml batch is a keeper I will then mix up 30, 60, or 120 ml, depending on how much I like it.
Sometimes, for recipes I know I love, I mix up a vial of flavor only (flavor shot, I guess you'd call it) and mark on the vial how many ml of
juice it will make. Then, all I have to do is mix up the base and dump the flavor vial into it next time I run out. This can save time as most of my mixes contain small amounts of 7 or more different flavors.