You're either using some form of unarmed combat to subdue them (use appropriate unarmed combat skill), killing them with a knife (use appropriate armed combat skill), or else applying chloroform, which would use whatever poison use skill is in the version of RM you are using. As for the hand over the mouth, that's just to keep them from crying out (unless chloroforming them, where it also does chemical delivery), which is covered by Silent Attack skill (folded into Ambush in RMU). You will, of course, need to sneak up undetected first.