Ointment

Practically I feel that there are three categories that apply:

Oil based semisolid ointments

Oil and water based (either lotions or crèmes)

Water based semisolids - a poultice

The simplest ointment to make is mixing the finely powdered herb with petroleum jelly. Some herbalists believe that the petroleum jelly should be heated and kept on a low heat while adding the herb and then forced through a filtering medium to remove the herb. That may work with some of the herbs where the active constituents are lipophilic (will dissolve in oil) but if they are hydrophilic (only water soluble) you have just thrown the healing power away and you are left with petroleum jelly that contains some contaminants. You are far better off leaving the herb in the jelly, at least it is all still there!

Another way is to buy a commercial skin lotion and mix it with a teaspoonful of the tincture (again, beware of dosage, just because you are putting it on your skin does not mean that dosage does not apply).

You can make your own by blending oil and water (sweet almond oil and distilled water will do) by using an emulsifier like lecithin (a bit like making mayonnaise by blending oil an vinegar, using the yolk of a raw egg as the emulsifier). More water less oil will render a lotion and less water more oil a crème. 

In some cases it is possible to use a filtered infusion instead of water - but again be aware of the dosage constraints of the finished product. Just because you put it on your skin does not mean that your body does not absorb the herb - otherwise why do it?

A lotion or a crème makes much more sense to me as that is capable of absorbing both hydrophilic and lipophilic constituents so at least you know that all of the herbal remedy is dissolved.

Store in a refrigerator. Commercial crèmes and lotions contain less volatile substances as well as a preservative, which is why they seem to survive indefinitely on the bathroom shelf.