Wash your face with warm water and cleanser. Remove all dirt and makeup. This will protect your skin and help the mask adhere.
Hold the mask up to your face to make sure it fits. Use a mirror to make sure you're positioning the mask correctly and that it looks the way you expect it to.
Apply liquid latex to the mask to use as a glue. Apply dots of liquid inside the mask at points where the foam will make contact with your skin. Use more liquid latex if you want the mask to be firmly attached for a long time. A couple dots on key points will give you a light hold (good for some quick photos or a short scene in a play), while dotting all over the inside of the mask will make it stay on for hours, even in the face of rigorous activity.
Attach the mask. Watch yourself in the mirror to make sure you're lining it up correctly and that the mask is straight. Press it to your face, then press your finger over the mask in all the areas where you know there's glue.
Seal down the edges of the mask. Apply a small amount of latex to the edge of the mask all the way around and press to seal it to your face.
Use makeup to color the uncovered portions of your face. Apply makeup to both your skin and the edges of the mask to blend the colors visually. Use transparent powder over all makeup to help keep it from running.