A couple hints ---
One tip before you start >>> drill a small hole through each horn on the back side near the base (this will give you a reference point to insert a nail later to line up the sheath where it was originally on the core). When the skull is clean and you're ready to slap the horns back on, car bondo will work its magic (as stated, the nail will be your guide for positioning the sheaths correctly before the bondo sets up).
DON'T boil to get the horns off the core --- you'll have ALL sorts of issues at the bases you don't want to mess with. If you put the fleshed out skull outside in a black plastic bag, the sun will heat things up in a couple days and, voila, the horns should pop right off. A caution here --- the stink will be considerable when you open the bag, so do so outside! Then clean away --- the skull MUST have all fleshy matter removed!
You want the insides of the sheaths as clean as possible before you reattach them to the cores. After you've removed the sheaths, fill them with baking soda and let them sit for a few days (just NOT out in the sun where they'll have the chance to shrink). You can then dump the baking soda and brush/blow out.
And as Randy stated, DON'T delay too long --- the sooner you tackle this after getting home, the better ...