Flavorings are usually PG based. you may wanna get some unflavored VG to mix with the flavorings. And maybe some unflavored PG. you also need to know what kinda VG/PG ratio you want. Most start off with 50/50, 20% flavoring, and adjust from there.
eg:
Watermelon (PG based): 2ml
Unflavored PG: 3ml
Unflavored VG: 5ml
total: 10ml, 20% flavorings, 50/50
Then just add a few drops of menthol concentrate to that.
Higher PG = more flavour, less clouds
Higher VG = more clouds, less flavour.
This program might help with the calculations. Also comes with user submitted recipes.
eJuice Me Up - Best eJuice Calculator