The flavours that are used by a lot of the e-liquid manufacturers or custom makers typically come from either Lorann or Perfumer's Apprentice (
Create your own perfume with The Perfumer's Apprentice) ...or some other reputable source. A good flavour base will make or break your mix. Some vapers find that the Lorann flavours tend to have a bit of a chemically taste, and I can actually speak from experience that this is true, as I've tried a few of them. Perfumer's apprentice flavours are much more expensive, but they tend to better.
Some PG or VG base also have a slight chemical taste to it that can be frustrating. Finding a nice PG or VG to mix with is one of your first priorities too, one which tastes clean and pure. You may also want to try out PEG400, as some vapers find that a 50/50 mix of PEG400/VG tends to give good vapour, but maximizes the flavour and throat hit too.
In the end, it's a matter of trial and error. Patience is needed, and a long period of investment to find the right mix and materials. If you persist, the rewards are good, as you get good taste, at a reasonable cost.
Another thing you may want to keep in mind these days, is that if you are using nic juice, some nic juice makers and sources have been exposed lately in other countries to either have less nic in the juice than claimed....or even more dangerous......much much more nic in it. There was one case highlighted her in ECF that a nicotine base that was claimed to be 100mg, was actually 272mg. THAT IS STUFF THAT CAN KILL SOMEBODY !!!! Even touching a 272mg juice can make you very very sick. Some nic bases were tested to show less than half of the claimed nic content on the label.
THIS IS NOT TO SCARE EVERYONE FROM MIXING YOUR OWN E-LIQUID...It's just to educate, and to make sure that you purchase your materials from reliable sources...and you do it safely....and end up with an e-liquid that will taste good and is safe for consumption. Mixing my own juice is something I can't do well, but I do admire the people who persist and manage to come up with some that they like.