How prize cards work:
-Each player puts down the top six cards of their deck as prize cards. They don't look at them or anything like that.
-Once a player KOs another player's Pokemon, they grab any one (or two if the Pokemon they KOd is an EX or zero if you faced one of the Shedinjas or a Pokemon with Life Drops attached) of THEIR OWN prize cards. They do not grab the prize card of the person whose Pokemon they KOd. They put this prize card in their hand.
-Once a player gets all six of their prize cards, they win.
So a player grabs one of their own prize cards. You could always place a sort of bet, having a card you give to your opponent if they win. I usually had it so that we could each choose a Common card of our choice if we were to win but that isn't an official way to play.