I've done it this way before now:
- Get some Glycerine, USP from a pharmacy.
- Dilute it 15% with DW (distilled water). If you don't have any then use ice from on the walls of the inside of a freezer, it's DW. Collect it by scraping it off very gently with a wooden spatula onto a saucer. You can create freezer ice by propping the door open a millimetre.
- Add enough flavoring to this diluted glycerine that it vapes OK. You can just drip the flavored glycerine into an atty, adding flavor until it tastes right (this is just a normal zero-nic vape).
- When you have your diluent right (the 0-nic stuff you're going to add to dilute the original), then mix it in the correct proportion. To halve the nic strength is easy: mix in the same amount of diluent as the original, for example 5ml of each, and you get double the quantity at half the original nicotine strength.
- Don't ever do any e-liquid mixing of more than 5ml at a time unless you are an expert. It's the fastest way to create a large amount of useless, expensive drain cleaner. 2ml is a good test amount when you don't know what will happen.