Allow the shade to dry. Sometimes what you are seeing is just the wetness in the satin, not an actual stain. If the shade is dry and the stain is still visible, it must be washed.
Use a clean sponge dipped in a solution of mild laundry detergent and cold water to wash the shade. Wash over the stained spot and around it. The stain is usually embedded dust or debris that was flushed out of the shade.
Hold the shade under cold water to rinse. Your shower or kitchen sink sprayer is ideal for this.
Let the shade dry completely before hanging.