bring the pack voltage to 0 with some sort of discharger (if u dont have, automotive 12v lamps will do the work). Use some sort of reading device to make sure the voltage is really 0! This is important, because if u have some energy in there u will kill whats left of the battery and possible harm yureself! then short circuit the batterie by connecting the plus and minus plugs.
I think there is some better explanation about this technique on the smc site.
Personally im not a fan of dead shorting because i feel it can reduce battery life. Besides i run modified and there is no big advantage there. But yure batterie is as good as dead, so no big risk here.