Once upon a time, Rubylith was fused into the silk screen mesh with acetone, a messy and highly toxic process. No more. Instead, you can use it to make a transparency.
Decide what you want to use Rubylith for. Rubylith will prevent light from exposing photo emulsion - in other words, Rubylith blocks light.
Choose Rubylith for large flat areas, chunky line art, or flat color areas - depending on your skill with an X-Acto knife.
Place the art you want to transfer into a silk screen print on your work surface.
Place a sheet of Rubylith (or Amberlith) on top of the art with the acetate backing facing down.
Use the X-Acto knife to cut lightly into the red layer - not into the acetate.
Peel the red layer up wherever you want light to go through, leaving the red (or amber) wherever you want the light blocked.
Use the transparency to expose your photo-emulsion-coated silk screen.
If you're worried about damaging your art, use a Sharpie pen to trace onto the Rubylith and then move the art before cutting. Think of the Rubylith as a positive mark: anything that's "ruby" now will be "ink" once you start silk-screening.
Don't cut yourself. If anything moves, you'll have to re-register it - so taping your art and the Rubylith to your work surface is not a bad idea.