Combine shea butter and coconut oil in a heat-safe bowl. Heat a couple of inches of water over low heat in a small saucepan. Place the bowl on top to melt the oils.
Combine arrowroot powder and diatomaceous earth in another large bowl.
Pour melted butter and oil into the bowl with the arrowroot and DE. Add in essential oils and stir to combine.
Stick the bowl into the fridge. When it's almost all the way solid, take it out and whip with either a hand mixer or emulsion blender. Keep going until the mixture is fluffy. Make sure nothing is stuck to the bottom of the bowl.
Transfer to a jar with a lid and keep in a cool, dry spot.
Apply a small amount to your underarms with your fingers.