If you want medium nicotine that's about 11mg per ml. If you start with a commercial eliquid at say (to make the maths easy) 22mg strength (they're usually something like 36, 24, 18, 16, 11, 9 and 6 mg nic per ml). Just dilute it by 50% to get from 22mg to 11mg. You can use glycerine, propylene glycol, small amounts of distilled water and food flavourings to dilute it. The final nic level will depend on what mg you start with and how much you dilute, you can make whatever strength you like up to the max of your commercial juice level.
If you start with 10mg juice and add 10% glycerine you'll end up with 9mg juice, etc.
Glycerine tends to smooth flavours so you might want to experiment with food flavours if you use that. If you like a harsh throat hit then you might be better with propylene glycol ... I haven't tried this much myself so I can't say how it affects flavour. It's a matter of trial and error until you find the quantities that suit your tastes. Some flavourings are stronger than others, LorAnn candy flavours are concentrated, ordinary food flavours may need to be used almost neat in some cases to give a good strong taste.
Be sure to use ingredients that are as pure as you can get. Pharmaceutical or food grade glycerine, not cosmetic stuff; flavourings without sugars, oils and colourings.
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