Experience and extensive trial and error.
I did try making my own juice for a while. I like blackcurrant flavours and assumed all I had to do was buy a Blackcurrant flavouring and, once I found out what ratio of blackcurrant to add to VG/PG, would end up with a juicy, refreshing blackcurrant flavour juice, just like the stuff I was buying at my local shop.
the results were pretty awful... It was definitely blackcurrant flavoured, but it was so dry I had to drink water constantly and the flavour definitely had something 'missing', quite frankly it had a lot of something missing.
I tried mixing from recipes - one I tried was Peach Custard. 'Common Sense' tells me this recipe would be Custard and Peach Flavourings. The recipe had Custard AND equal amounts of Bavarian Cream - it did include Peach flavouring - but it was equal amounts of two different types of Peach flavours and, for some reason I couldn't fathom equal amounts of Strawberry flavour, and Vanilla... i can't remember the entire recipe, but it included 8 different flavourings - six of which i wouldn't have expected to be part of a peach custard juice. Yet, after 6 weeks steeping time, this juice was excellent - and it improved with age.
Sadly I never managed to make a fruit flavoured vape that even came close to the 'professional' store bought juices - and following other people's recipes seldom worked for me. Some I could vape - but I never found one I would buy from a shop if any of my favourites were available.
Making good juice requires knowledge and skill. Sadly I don't have either.
