The best way is to hang them from a fence out back in the farmer's property, where the birds and ants will clean it out.
However, you probably want to do it yourself. SO-O-O-O, OUTside on an open fire, or a BBQ grill, boil a large pot of water, put some pine sol in it to improve the odor, and maybe even some Lysol, and drop the horns in the water to boil. That will loosen the core, so that it can be pulled out. Wear gloves to protect your hands from being burned by the hot horns as they come out of the water. Until the cores are removed, the cores will retain heat and moisture for a long time.
You will want a large wood screw to turn into the softened core, to pull them out, and you will want to stay upwind of the boiling. This process stinks. Once the cores are out, put them in a plastice bag, like a bread wrapper you were going to throw out anyway, and put them in the garbage. Then dump the water, to help get rid of the odor.
Use fresh water, and use it to loosen any tissue that remains, so you can scrub out the horns with a brush. I like those brass bristled brush sold at craft stores, because the bristles are strong enough to remove sinew, but soft enough not to scratch the horn too much.
Let the horns dry, and then scrape off the scales on the outside to get a smooth finish.