Choose three foundations in the color closest to your skin color. The foundations should have different undertones, such as cool, warm, and neutral. Cool would be pinkish, warm would be more orangish, and neutral would be a slight yellow to beige.
On a clean jawline, place one stripe of each color along the jawline at the side of your face. As you apply each color tone you will be able to see two very well and the third should disappear slightly into your skin. The one that disappears more will be the correct undertone for your skin.
After finding the correct undertone, you can begin finding the correct shade. Keep in mind that the correct shade will completely disappear onto your skin, so you will not have the look of wearing a makeup mask.