With any luck your fabric of your costume was pre-treated (pre-washed) before it was built as your costume. Chiffon can be a blend of fabric content. Chiffon silk or chiffon polyester...etc etc. Silk reacts differently than polyester. Plus the color may run/bleed/fade and the fabric may react as well. You might "test" some underside of the skirt taht is the little seen part of the skirt fabric - that doesn't show to the audience. To see how it reacts with handwashing/cold water/and air dry or laying it flat style of drying before attempting the rest of your costume. Everything you do in washing your costume will be gentle. IF you make the leap to wanting to wash/clean the entire costume find a net/mesh laundry bag for delicate clothing (ususally sold in laundry aisles where I live of our local stores) where the mesh is so fine it will catch the stones that go missing in action during your cleaning. Still, cold water and gentle detergent/hand wash gently, if this is the route you decide on. I use Gem-Tac glue to replace the stones back onto my costumes. This advise is not for everyone - it is simply what works for me.