I have used it both ways. When you mix it ahead of time there are two things to consider. The solutions are cold, so you would have to be OK with pouring cold water on your head. I use Bragg’s Organic Apple Cider Vinegar with the Mother, and if the solution is left in a container too long it forms a mold. Do not ask me why. I was surprised to.
Also you would want to use a bottle or container that does not leech plastic.
I know it seems a little much to mix everytime, but that is the way I like it best because I can use warm water. I have been doing this for 8 months and the difference it has made in my hair and scalp is so worth the effort. My scalp does not itch and always feels clean.
One more thing. You have to play with the amount of baking soda to the eights ounces of water. If you hair tends to be oily, use more. If it is dry, use less. At the time I started this, I had color treated hair and it still worked like a charm.
One more benefit, it so Inexpensive!