2. Inhale quickly with your nose to create underpressure.
3. If the mask stays on even if you shake your head a little bit, the mask fits you.
The lower end of the mask should not reach your upper lip, and the upper end should not be too high on your forehead. The width should fit perfectly your face, it is not supposed to pinch or be uncomfortable. The lens should be as close to the eyes as possible, but there shouldn't be pressure or pinching on the face.
Please notice that if you have mustache, the mask probably doesn't tighten properly. Make sure there is no hair between the seal and your skin. In use, the strap shouldn't be too tight nor too loose. If the strap is too tight, it flattens the sealing rubber / silicone and the mask is uncomfortable.
If you are buying your first mask, we recommend you to try on several masks in our store so that you can be sure your mask fits you. A mask that fits your friend doesn't necessarily fit you.