I just use a liquid detergent that is intended for use on pure wool sweaters (for example, in Canada, one brand is called 'Woolite'), and I throw the sheepskin into the washing machine, put it on the gentle cycle, pour some detergent in, and let the washing machine do it's thing. That has always worked well for me.
When the wash is done, lay the sheepskin out somewhere (ideally, outside, in the shade) to dry. They tend to stink until they dry out. When it is dry, you can use a hairbrush to untangle the 'Rastafarian' look that the sheepskin will have after having gone through the washing machine cycle.