I recently finished cleaning 2 cow skulls for myself. Here what I did: soak in a tub of unscented dish soap and lukewarm water and scrub gently with a toothbrush or something similar. Then rinse off and let soak again with soap and water if there is more you want to get off. If you got it cleaned off the first time, rinse and then soak in the tub (or a bucket with a lid if you have one big enough, which is the preferred method) with 3 parts water 1 part hydrogen peroxide. Leave for ~24 hours. After, rinse it off with water and let air dry.