Before You Start Mixing

Before you start mixing, you need to understand the importance of safety and risks using these compounds.
 
While vapor has been said to be a less harmful alternative to combustible tobacco, it is not without it's potential dangers. There are always risks any time you put a foreign substance into your body. If you are concerned about these risks, I suggest not vaping or smoking. For those of you who want to get away from combustible tobacco, and haven't had much success quitting cold turkey, or quitting with other methods, vapor may be a solution.
Nicotine
Most people who vape add nicotine to their e-liquid. However, it is absolutely not required. In fact, the goal of many vapers is to lower the nicotine levels over time to become completely free of nicotine. 

When working with liquid nicotine, you must take caution! Nicotine can be highly toxic in concentrated form.

 
Nicotine for mixing e-liquid comes in dilutions of 100mg/ml or less. This is referred to as 100mg NIC. I suggest NEVER to use any nicotine dilution higher than 100mg/ml.
 
When working with nicotine, always wear gloves. If you happen to get it on your skin, wash it off immediately. Always store your nicotine away from children and pets. If you follow these simple rules of thumb, you will be able to mix your own e-liquid with nicotine safely.
The Flavorings
 
Flavorings, in my opinion , are why there is a high success rate when it comes to getting off combustible tobacco. Why wouldn't someone want to taste a sweet delicious custard or fruit as compared to the ashy, stinky of cigs?
However, these compounds as with any substance, can be potentially harmful. Although the FDA has approved these flavorings to be "Generally Recognized As Safe", we don't know the long term effects of vaping these compounds. So if this concerns you, I suggest not vaping. However, if vaping is your best solution to getting off combustible tobacco, then you must weigh the risks for yourself.

Vegetable Glycerin and Propylene Glycol

Vegetable Glycerin(VG) and Propylene Glycol(PG) are the main ingredients that are used as a base for e-liquid. VG  is derived from vegetables. It is thicker than PG and is often used as the cloud production aspect of e-liquid.

Propylene glycol (PG) is an almost odorless, colorless, synthetic liquid with a slightly sweet taste. It is thinner than VG and is used in e-liquid as a flavor carrier. PG is classified as a food additive generally recognized as safe by the Food and Drug Administration.