u can gas them with CO2 or dry ice.
if u don't have those means, u have to get hands on and perform cervical dislocation. it is important that u get this method right to make the death as painless as possible to the rodent. (and least traumatizing to u)
this guy does it perfectly. he euthanizes dozens of mice efficiently and painlessly.
unless u are gonna offer up the freeze kill, pack them and freeze them.