Soda ash (sodium carbonate) is used to ‘fix’ dye to clothes and make it permanent. It does this by changing the pH of the fibre reactive dye and cellulose fibre so that the dye reacts with the fibre and makes a permanent connection.
You can add the soda ash directly to the dye, however I prefer to presoak my garments for 15-30 minutes before dyeing. I use a solution of 100g-130g of soda ash per 10 litres of water dissolved into a tub. This solution can be reused, so as long as you have somewhere to store it you can use it again and again until it runs out. Once you have finished soaking you can spin the excess liquid out in the washing machine, or wring it out by hand.