Mix all of the ingredients in a glass bowl (do not use metal!) with a wooden or non-metal spoon until about the consistency of sour cream.
Spread in an even layer over the armpits and allow to sit for 5-20 minutes. I suggest starting slowly and working up to a longer period of time once you see how your body adjust. If it hurts at all, remove immediately.
This may cause some redness as it will increase blood flow to the area, but it will go away quickly.
Wash off in the shower or with a warm, wet washcloth.
Repeat daily or as needed until underarm odor goes away and natural deodorant is non-irritating.
I've found that I don't even need the baking soda in my natural deodorant after doing this.