Mix the magnesium powder into the water, making sure there are no lumps.
Add your ice to a glass. Pour your tart cherry juice and magnesium mixture over the ice. (if you'd like you can add these ingredients to a shaker, but to keep it simple you can pour it right into your glass.)
Add lime juice and give it a stir.
Garnish with a lime slice and enjoy! For some fizz, feel free to top with sparkling water.