Heres how I did it:
Darkstar for PG, VG & Nicotine (cheap & good quality)
Leisure Liquids for Syringes & flavor concentrates. (loads of variety you can order at once)
Amazon for a job lot of 30ml bottles
I picked up about 15 simple concentrates & a couple of premixed flavors, then mixed a 5ml batch of every flavor. Left them to steep for 2 weeks (after using the tumble dryer trick to thoroughly mix them
).
Once you have tried all the simple juices on their own you can see if they need longer to steep, or more flavor, or if you don't like them at all.
If you like them as they are, then you can mix up a larger quantity of the flavor. (I like my watermelon simple & heavy on the flavor)
Now know what the flavors taste like & can start with simple mixes. e.g. cherry choc, or vanilla custard with strawberry. (start in small batches as above to see how you like the combinations)
Then the world is your oyster.
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Two tips that I have found from my experience of doing this:
1. Have pre-mixed juice ready to use while you start playing with DIY (you don't wanna be caught without anything to vape while you wait for your flavors to steep.
2. Buy a cheap scale rather than syringes (I found the syringe really messy to use).