This stuff really works! Keep in mind, this is not antiperspirant, you will still sweat. Also, the coconut oil will melt in hot weather. Don't leave it in your backpack on a hot day. I learned that lesson the hard way.
Melt about 1/3 C coconut oil over the stove or in the microwave. This amount is just a rough estimate. Your quantity really just depends on the size of your storage container.
Remove from heat and mix in baking soda until you have a very thick paste. You should be nearly unable to stir the mixture. Add enough tea tree oil to cover the scent of coconut oil. Add any other essential oils that you enjoy-I love lavender or patchouli ;)
Pour contents into your preferred container (I just re-use an old deodorant container) and you're done!
Basically, the baking soda and tea tree oil are eliminating the odor. The coconut oil is just there for easy storage and application.
You can get coconut oil in most health food stores.
A note on coconut oil. Unrefined coconut oil will have more delicious nutrients for your skin, but will also have a stronger smell than its refined counterpart. Choose whether you want less scent or more healing properties when selecting your coconut oil.
Also be careful with any essential oils you use on your body. Do some research to ensure that they are non-irritating. I always go organic with any EO used for personal care. I do buy cheaper oils for cleaning products.
Mountain Rose Herbs has great info on their site and carries high quality oils.
Wholesale Supplies Plus has great prices and selection: