Dharma Fiber Reactive Dye must be 'fixed' (made permanent) with soda ash on cotton and other cellulose fibers. It's a mild alkali that enables the reaction between the dye and the cellulose fibers at lower temperatures, causing them to "become one", and the reason Fiber Reactive Dye is so permanent. How much you use is normally a function of the amount of water, not the amount of fabric or dye. Roughly 1 oz. per gallon of water is needed when garment dyeing or 1 cup per gallon of water as a pre-soak when tie-dyeing. If you don't use enough, the dye won't yield full strength. A pH of 10.5 is ideal. It can also be used to fix fiber reactive dyes on silk when you don't want to use a lot of heat (as with steaming or the simmering with vinegar method), but be careful, as it can reduce the sheen, or even cause silk to fall apart - use less, and expose the fabric to it for less time than cotton.
Caution! The soda ash solution is mildly caustic - it can irritate your skin.
Wear gloves and eye protection. Rinse off at once to avoid eye and skin irritation.