Decide the style
Decide which style or era of gas mask you want. Human beings have been using gas masks in dangerous situations over the years. Sketch the basic pattern of the mask you want to make.
Base of mask
Use any old hat or the inside of a helmet to make the base of your gas mask. You can also make your own base using hard straps which fit around your head.
For the eye pieces, find anything transparent, making sure that you can see through the lens clearly. Tint the eyepieces using a car window tint film. Make two short cylinders using a stiff but flexible material, like acrylic or plastic wrap, and use them to provide a cover to your eye pieces. If you fail to find a suitable material for the lenses of your gas mask, substitute them with a pair of goggles.
Use another cylinder to make the mouth piece. You can use anything like a painted Styrofoam or plastic cup, a can to make the mouth piece. Affix the mouthpiece to the base of the mask using glue.