I learned by following the KISS method. (Keep It Simple Stupid)
I started by learning the few basic single flavor mixes I like. Dealing with only a single flavor is a lot more forgiving. You simply start a bit on the weak side and add flavor until it's right where you want it. Once you've got that nailed for the basic flavors then move on to double and maybe some triple flavor mixes using the single flavors you already know.
For example, I love peppermint so my first DYI project was nailing down a basic strong peppermint that I really liked. Once I got that where I wanted I decided to add some cream to make a lightly creamy peppermint candy cane flavor. Once I had that where I wanted I added a touch of cherry and then a touch of vanilla. End result was a wonderful cherry peppermint candy, but I learned it in simple steps, each flavor one at a time, not by diving right in to try to get the 4 flavor mix right from the start.
Another thing that helped me was rather than mixing the flavor concentrates I'll often mix finished juices to get the flavors I want. Mix a batch of vanilla, mix a batch of peppermint, mix a batch of cherry, and then combine them rather than the concentrates. That way the PG/VG ratio and nic amounts never change. Now matter how I tweak the flavorings, adding more of this or that, the finished mix is always the same PG/VG and nic level without having to go back to a calculator and remix anything.