Soda ash changes the pH of the fibre-reactive dye and cellulose fibre so that the dye reacts with the fibre, making a permanent connection that holds the dye to the fibre.
Adding the soda ash before the dye is the usual method for tie-dye. Make up a solution of 1/2 or 1 cup of soda ash per gallon of water (soda ash dissolves best in warm water, 96°F. or 35°C.), and soak the material from 5 minutes to an hour, depending on your recipe and materials. Then, wearing gloves, wring out the material, and prepare to apply the dye.