You need a few things besides just the latex prosthetics you're going to apply. I was clueless and time crunched enough that I just bought the recommended adhesives and removers and makeup at the costume shop. You may prefer to be a little thriftier.
- A pair of elf ears
- A very small amount of spirit gum (Structural)
- Spirit gum remover (Any oil you're willing to rub on your skin)
- Liquid latex (To smooth the edge between prosthetic and skin)
- Makeup (You'll need special makeup to put on the latex. Normal makeup will break down the latex, and not stick, and you'll panic thirty minutes before your event. Don't laugh, it happened to my friend.) (The makeup we got, Ben Nye, was very very stiff. A liquid would have been nice for basic coverage, at least.)
We didn't have the forethought to photograph all of the supplies since we were frantically applying four people's ears two hours before a wedding that was an hour drive away.
According to timestamps on the photos, I was able to complete one pair of ears in 45 minutes, including drying time, but I would allow two hours for your first attempt. Liquid latex can cover a multitude of sins, but it takes time to dry.