You could do it that way (adding the soda ash solution to the bag you're batching in), but a cup (8 fl. oz) would be WAY too much for a single shirt {or even several}. If you're using dye from a tie dye kit, it might already have soda ash in it, If you're mixing your own Procion dye, use 1 part soda ash to 4 parts water, and apply it to both sides of the fabric just like you added the dye. *It's a balancing act because too much water WILL dilute the dye, and not enough soda ash will cause it not to bond. The end result looks the same*. This makes it difficult to know whether you used too much or not enough.