Traditionally the ears would be sculpted in clay, a mould made and the ears cast using foamed liquid latex. This can be long winded, technically challenging and expensive due to the sculpting skills, additives and baking required. Also once the mould is made it can only produce that design of ear.
Here's my take on a slightly different approach, using two kinds of armature. One type will allow you to make any design of ear you like, but requires more work at the fabricating stage. The other requires more work on the armature but can then produce lots of sets of ears reasonably quickly.
All these ears rely on the slip latex or skin latex process. This requires no additives or baking at all.
The two images here show the small ears fitted but without make up added, and a pair of the long custom ears as part of a Dark Elf body painting.
Before you begin please be aware that these ears are made of natural latex. Some people are allergic to latex either in it's liquid or cured state. Most people over the age of 18 would be aware of any allergies, but to be sure do a very simple test on the back of the hand.
Using a small amount of liquid latex and a small sponge dab a thin layer about an inch or 25mm in diameter onto the back of your models hand. Allow a few minutes for this to cure (it will go transparent). Apply a second thin layer and allow that to cure as well. If after this time there has been no reaction then it is very unlikely that there will be problems. Gently roll an edge with your finger to lift it and peel off the test patch.
Some people with very sensitive skin (those who have difficulty finding suitable make-up for example) may show a very slight reddening and mild itching of the area. This is normally not an allergic reaction, but a reaction to the very small percentage of ammonia present in all liquid latex (usually less than 0.5%) used to prevent the latex from curing in the bottle. The ammonia will evaporate as the latex cures.