I use Walmart's Humco glycerin.
You don't have to mix it with anything... Glycerin will wick better if it's thinned, though. PG or distilled water will work fine.
If you'e going to add it to a premixed e-juice, then it will lower the nic and flavoring by whatever ratio you mix it in at. If that's the case, 20-30% would probably make a good difference.