One drop of concentrate can make all the difference, and if you put in one drop too many, it can turn a delicious tasting vape juice in to a bitter mess. Start off with small batches, and make sure you get the ratios right. Too much concentrate can make the liquid unvapable, so use small amounts to begin with.
Finally, use a e-liquid calculator to make sure you are using the right amounts of each ingredient to get the nicotine strength and ratios to your preferences.
After a few hundred attempts, you should come up with a recipe for success!