With your fingers putting pressure on the top corners of your mask, look up towards the surface. Take a regular breath from your regulator and exhale through your nose. Your mask should clear.
Don't be discouraged if your mask doesn't completely clear on the first attempt - just take another breath and try again.
Remember: If your mask isn't clearing you may not be exhaling through your nose. Many people confuse exhaling through their mouth and nose. Try to concentrate on breathing out through your nose or try placing your tongue on the roof of your mouth as you exhale (this forces most people to exhale through their nose).
Once your mask is clear, breathe normally and give your instructor the OK sign.