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A simple and inexpensive way to make a form for you mask is to use a balloon.  The balloon is inflated to a size that is roughly that of your face.  The paper mache will be placed over the balloon which is later popped, leaving you with only your mask. For a simple mask you can make use of cardboard or poster-board pieces to build the mask's form.  Cardboard or poster board can be a great choice for your mask form, allowing you to create a stable shape that you find interesting.   Cut out a flat piece of cardboard or poster-board that roughly matches the shape of your face. Cut out smaller pieces and glue them to the first flat piece to build features like noses or mouths. If you want your mask to be highly detailed and have complex shapes, using paper and tape to build a form is the best option.   On top of a flat mask base made from a piece of cardboard, paper is wadded up into bundles and taped down, forming any shapes you might want.  This method allows you to make detailed and complex shapes. Smooth shapes are possible to create with this method as well. Try to line up your masks eyes, nose and mouth with your own so that you can see and breath out of it once it is done.  Hold the mask form up to your face to get a rough idea of where these features should go.
Find a balloon to use for your form. Get a piece of cardboard or poster-board. Use paper and tape to form the mask.