At the beginning of a recipe that requires buttermilk, add the lactic acid to the coconut milk and water (or your milk of choice) and stir well to distribute the powder thoroughly. Set the mixture aside until you're ready to use it, at least 15 - 20 minutes.
*You may wish to add more water and use less coconut milk. If your recipe can withstand additional fat, go ahead and just use 1/4 cup water and 3/4 cup coconut milk. Otherwise, you can use 1/2 water and 1/2 coconut milk. Full-fat coconut milk is about 1/2 fat. So, adding 1/2 cup of buttermilk is like adding more than 1/4 cup of butter to your recipe.