Well, what I can tell you right now are a few things:
1: Let them steep!!! Mostly will come all right by the time you get them, BUT, if you don't like them (because I know I did for a bunch of samples that I bought) it might be because they are not ready yet. Just leave them alone in the dark for a week or so (probably up to 3 weeks for sweet custard type of juice), shake them once a day and let some fresh air inside the bottles before shaking them
2: If possible use a dripper, I bought some cheap T3S to test new juices and I got a few brunt wicks in them, after that, they all tasted AWFUL! but I left them to rest, used a decent tank (nautilus and aerotank) and flavor and taste came through!
3: If you already had a juice that you like, use the same pg/vg ratio for now, Try with smaller samples before changing your ratio
4: Freshly made juices with nic on them will taste like perfume (at least to me, I have a very sensitive nose and taste buds) that;s why you let them steep, and nic % will sting (to me) on fresh juice, after a week or 10 days everything is all right.
5: Don't switch one juice after the other, I find that one flavor will saturate my taste for it, I cannot perceive fully the next one until a while has passed, there are a few tricks to speed things out, but I like to take it slow, for me, I give a new flavor a full day or until the sample runs out (3 to 5 ml bottles) and then move on
Hope it helps!