Buy a quality spray fixative from your local arts and crafts store. A workable fixative allows the artist to spray and continue drawing over the spray, while a non-workable fixative doesn't.
Test the spray fixative with a separate throwaway drawing before applying to your good works. This helps you get a feel for method and application.
Consider experimenting by applying a topcoat of fixative to blank drawing paper or some other medium. Pastels affix well to this initial coat.
Spray a light mist to contain the upper layer of pastels. Fixatives reduce smearing and protect different layers from mixing when the work needs several layers of medium.
Seal the pastels, pencil, chalk, watercolors and charcoal with a final spray. Secure the drawing outside to a table or inside in a ventilated room.
Apply quickly, without saturation, from about a 30-degree angle about 1 foot away. Read the can's label for various distances. Spray the fixative horizontally across the piece and let it dry. Then apply it vertically and allow that to dry.