Develop an understanding of the various types of oil before you make the decision of buying one. It's necessary that you should be aware of what you are interested in buying beforehand, instead of going unprepared looking for that something that you don't really have an idea about. Oils are an important component of cologne therefore you need to have information to get yourself the right type of oil that would serve to be the perfect ingredient for your product.
Make sure that the oil you have chosen is the right type for making the cologne. There are two types of oils used for making up colognes, one being the manufacturer grade, while the other is the cosmetic grade. The first one is a more concentrated form, being more expensive comparatively to the other one. The cosmetic grade however has more preservatives and diluted solution, also being less expensive. Moreover, the cosmetic grade can be directly applied on the skin, unlike the manufacturer grade.
Once you've done with your homework and chosen the oil that you intend to buy, visit a store nearby that possesses a variety of perfume oils. These oils cannot be ordered online, as a person has to smell them in person in order to decide which fragrance better suites your choice.
Take some small cards along with you when you head to the store. Apply the oils on these cards and then write the name of the oil on them, so that you may remember which fragrance belongs to which oil. If you can't decide at that moment, don't rush into a decision straight away. Just get your cards and go home and take your time to make decision. Test how long each fragrance lasts, and the smell that you'd rather have as an ingredient.
Learn about the different combinations that could serve as ingredients for the perfume. Try different oils and mix them up together to see how it goes. Who knows maybe you'll just invent a secret ingredient in doing that?
Make the purchase when you're sure about what you want. The samples that you have tested, along with the different mixes that you have come up with would help you in making the final decision.