How do you properly wash your hands to kill any sperm on them?

If preventing pregnancy from finger contact is your concern, don't be worried. Even with no washing at all, there is little if any pregnancy risk. In any case, a brief water-only rinse with drying would be sufficient; a quick soap and water wash would also be enough to provide complete protection. (You may be the partner of a guy asking similar questions. Let's make this the last one, OK?)