I am not highly educated in either the use or application of cosmetics, so I encourage you to supplement my answer with independent research.The goal with foundation is to match as closely as possible to your natural shade. Select three that you feel are very close to your skin color, and apply a small amount from your cheek, over your jawbone, and on to your neck. Using a cosmetic sponge, blend all three into your skin. Then, select the color that most closely matches your NECK. This may seem counter-intuitive, but many ivory-to-beige skinned women have red in their face that is not present on their neck, and matching it to this undertone results in an incorrect shade.Furthermore, when selecting your foundations to test, look for those with a YELLOW undertone to them. Yellow will counteract the most common flaws in skin-tone, such as dark circles under the eyes, rosacea, etc.For women with bronze-to-ebony skintones, a relatively common phenomenon is “facial masking”, where the skin around the edges of their face is a shade or two darker than the skin along the face’s midline. If this applies to you, instead of making your streak from your cheek onto your jaw and neck, run the streaks of foundation from your cheekbone down to your jawline. Blend as above. The goal is still to match your neck, but this will help you pick a shade that works well with your natural variation in color. Unfortunately, selecting the correct undertone is a bit trickier with darker skin colors; disregard the “go for yellow” advice above, and try to select the foundation that seems to match your undertone the best.
In all cases, do your “Streak test” in natural sunlight. Take mirror with you to the department store and go outside to look, if you must.
In the event that you find your skintone is between two different shades of foundation, you should select the slightly darker shade, as the lighter shade is likely to make you look washed-out and ashy.