For the most basic cream all you need are three ingredients:
1/2 cup grape seed oil
1/4 cup ewax
3/4 cup water
From this basic formula, you can add all sorts of extras. The following are all standard ingredients, and won't add drastically to the cost:
1 tbsp glycerin (add this to the water)
1/4 tsp vitamin E
12-15 drops of essential oil
1/8 tsp of Germall Plus (a preservative)
Note on ingredients:
I wanted to avoid fragrance which meant I had to choose my oil carefully, since I couldn't use perfume to mask an unpleasant oily smell. I chose grape seed oil for all sorts of reasons: the smell is neutral, it gives a cream with a very pleasant texture which absorbs nicely without feeling greasy, it does not clog pores and give you pimples, it has a good shelf life, it is widely available from any grocery store and it is (relatively) cheap. Jojoba oil, for example, which shares many of the same qualities, is much more expensive. Castor oil is also odor free and has a great shelf life, plus it has the advantage of containing lots of ricinoleic acid. This compound has a useful anti-inflammatory effect -- but castor oil gives the cream an unpleasantly heavy, sticky feel if there's too much of it. I could go on and on about all the different oils... suffice it to say, after much experimentation, this one is my favorite. If my eczema test subject is suffering from an outbreak I might combine it with castor oil (I'll use 1/3 cup grape seed, then enough castor oil to get 1/2 cup oil) to help reduce inflammation.
Ewax is a blend of cetearyl alcohol and polysorbate 60. The E stands for emulsifying, and it's purpose is to blend the oil and water into a smooth, stable cream (you might find it under the name "emulsifying wax NF" in online stores). It comes in white, waxy flakes which are easy to measure and melt. I like it better than the "all natural" alternative of beeswax and borax because the cream is less sticky and more stable (i.e. the water and oil in the cream are less likely to separate). I've never seen this sold in a brick and mortar store but it is very easy to find emulsifying wax online. One pound will last a long time...
Although supposedly distilled water is better, try as I can I have never been able to discern any difference in quality between creams made with distilled water compared to plain old tap water. Save yourself the trouble and use tap water.
Glycerin is a by-product of soap making. It is a humectant, which means it attracts and absorbs moisture. Adding it to cream helps your skin absorb moisture from the air, but if you use too much the texture of the cream suffers, it starts feeling sticky. You can find glycerin or vegetable glycerin (derived from vegetable oils rather than animal fats) in most drug stores, or you can order some with your ewax.
Vitamin E (aka tocopheryl acetate) is also sold in drug stores, but usually it is not pure. They mix it with water or glycerin and other stuff... you can buy it pure wherever you order your ewax. I like adding it to any cream I make because it is a very strong anti oxidant. This means it will help prevent your oil from going rancid, but it will also help protect your skin against UV damage, bring nourishment to the cells, assists in the healing of damaged tissue and prevents scarring. However, it also prevents blood from clotting so it should never be applied to a bleeding wound.
Essential oils will give your cream a nice scent, and different oils are reputed to have various therapeutic properties. Others, such as any citrus essential oil and ginger will sensitize your skin to UV radiation, so avoid using them if you anticipate any sun exposure.
Preservatives are ESSENTIAL if you plan on giving your cream to someone as a gift or selling it. I speak from experience: I once gave some cream to my mother in law in an opaque pump container. She thought it was so special that she never used it and kept it in her guest bathroom. When I visited about six months later I was very glad to be the first guest -- the stuff was absolutely foul, black, smelly, disgusting in all ways, and since the bottle was opaque someone could have easily used it without checking... That said, I don't generally use preservatives for myself because I make small quantities and use them up very quickly. Instead of putting the cream in one big jar I'll use several small ones and store all the spares in the refrigerator. I also make sure I can see the cream before putting it on, and if hairy stuff starts growing or if it smells "off" I'll throw it out. That's doesn't happen often, and when it does it's not a big deal: at about $1.15 per 16oz of cream, I'm not loosing much. I have only experimented with Germall Plus (again, order it wherever you buy your ewax) because it is a paraben-free preservative which is easy to use, but there are plenty of others. This one works quite well though, I have a one year old test jar of cream still going strong.