Stir together Epsom salt, Sea salt, Baking soda, and peppermint extract (or oil).
If you want to layer pink and white layers of the salt to look like a candy cane – divide the mixture into 2 bowls. Stir 6 drops of red food coloring into one bowl. It will turn pink. You don't want it to be red as it will stain your bathtub (and probably you)
Choose a pretty airtight jar to store it in. If you want to layer it with two colors a taller thinner jar would work better.