To use a duo fiber brush, it's easy even if you are not accustomed to use a foundation brush!
1) Take some foundation.
2) Apply to the skin with little circular movements. No brush marks, your foundation is invisible! The constitution of this duo fiber brush (with 2 lengths and 2 different textures of hairs) blurs the foundation.
Last tip: always start with the center of your face and gradually bring the foundation outward for the most natural result.