Remove the can from the fridge, do not shake. Open the can and scoop up all the hardened coconut cream, leaving the coconut water behind. Add the coconut cream to a bowl with the powdered sugar (add to taste). Beat the coconut cream and powdered sugar together, whipping the coconut cream. You aren't so much whipping the coconut milk up, more like breaking it down. If it's too thick add in a splash of the leftover coconut water, or any non-dairy milk until you reach desired consistency.
Make sure you get coconut cream or premium coconut milk. A good test to see if the coconut milk is rich enough for this recipe is to shake the can. If you can't hear any liquid sloshing around, or very little liquid then that's probably a good brand. I recommend Thai Kitchen Premium Coconut Milk.