Mix the following:
1 part Shea Butter (use unrefined!)
1/4 part solid oil, such as coconut
a drizzle (to preference) liquid oil
Essential oil (optional)
Any cooking oils will do. You can go as cheap or fancy as you want to. I prefer cold pressed oils and Nutiva extra virgin unrefined coconut oil—I've tried them all, and this is the best—but you don’t have to. Buy the cheap stuff at the grocery store. Try it out, see if you like it. I'm sure you'll find that it’s a heck of a lot better than most of the other stuff out there.
What about shea? That you'll probably have to order online. Unrefined shea does have an unusual ash-like scent, but is packed with antioxidants that are chemically stripped from refined varieties. I think you'll find that the benefits of unrefined shea outweigh the smell.
I'll go ahead and assume you don't want any fuzzies growing on your butter, so please sterilize and dry your utensils completely before using them! Or at least make sure there is not so much as a drop of water in your container. If you don't care to serilize, most likely nothing will be growing in your butter unless you incorporate water in the mixture.
I use a stand mixer to blend everything (Kitchen Aid,) starting with the more dense shea butter, and ending with the liquid oils. It's a simple method that works well.
One little note: be careful what essential oils you use to scent this product. Clove oil, for example, contains high levels of Eugenol, a hepatotoxic compound that may damage the liver over time. Even lovely Bergamot is carcinogenic. That being said, I just noticed this morning that my natural Zevia soda contains Wintergreen - another toxic essential oil. A little obviously won't kill you, but please be careful.