There is a thread on this here on TAS - sorry I've never been able to find things when I search.
You need to get the core out of the horn, unless the men pulled them right off, but even then they will need a bit of weathering. You can boil them up in washing powder, but we leave ours on a shed roof, well away from the house, for several months for the weather and insects to do their job. At the end of that time they will be clean and dry, but you might want to scrub them out inside with a bottle brush.
If you want to keep them whole, polish the outside with clear oil. If you want to make something such as jewellery, slice and polish. They will not be big enough to make a crook, for that you need an old tup with big strong horns.