But how would you stamp a POW block which has already been mined?
50% of the minting & tx fees for block N would have to go to a valid stamp for block (N-3). If there was more than 2 valid POS stamps (meets difficulty), then the strongest one would be used.
The exact method would depend on which POS method selected.
Does POS mine txns in the stamping scheme?
No it is just POS validation of the POW chain. The purpose is to make it harder to do forks.