Bear in mind los8383, some flavorings such as tobacco absolute ,you will need less than 1% of total to flavor with , others 20% or more . Therefore , with 24mg/ml nic to start , it is hard to achieve target of 18mg/ml if you need 25-30% flavoring .
I have no experience with flavorings from Wizard Labs so I cannot comment on ratios . I personally , would get at least 36 mg/ml nic to start . You can vape that if you wanted to without risking death and have some leeway with differing flavor concentrations.
Buy the flavorings you think you would enjoy. Buy smaller quantities to start. If you are not allergic to PG , get 100% PG nic or 90/10 pg/vg to start with. Add vg (can be purchased at walmart) to dilute. PG carries flavor well and has a harshness (throat hit). VG tends to mute flavors , has a sweetness of its own (slight) and makes huge plumes of vapor.
Aim for a 70/30 % pg/vg ratio to start. Go up or down when you find what you like.
You are correct in that nic and flavoring in proper dilution ratios is pretty much it . Getting it to taste like a juice you bought and like is the hard part . Some add vinegar , some add lemon juice ,others ethyl maltol ,yet others sweetener ,menthol crystals. Some a combination of all the above . Taste is subjective of course. What someone else likes , you may find horrid.
Read, read , read. List what you bought in DIY forum .Ask for advice . Try said advice.Then ....give it a go. Adjust to your taste .Remember to write down what and how much. Many tales of the elusive recipe that is not repeatable do to poor/no notes.
If you want to not be running to Walmart and Tractor Supply (they sell PG by the gallon). Get your nic , some plain pg, plain vg , flavoring ,empty bottles , and a few 3-5 mil syringes to start. If doing tobacco flavors, get some ethyl maltol (cotton candy flavoring is the same thing). Ask questions about the calculator and be safe . Be patient . Making juice is frustrating .It is also rewarding . You can save money but probably won't for a while. I have enjoyed learning . So much left to learn.
I'm still a novice at DIY though I do understand the whys and the hows. There are some very helpful vets in the DIY section willing to share their knowledge.
You are welcome and good luck.
Russ