Add in the magnesium flakes and salts. Shake, shake, shake!
Then, add in your essential oils and shake some more.
I use about a 1/2 C for each bath he takes. He's about 30 lbs and doesn't have much water in the tub. I use it every time he bathes, which is 3-4 times a week. Feel free to double or triple your batch so you won't have to make it so often.