Cashback is a slot club benefit, not much different than a comp to the buffet or coffee shop. To collect cashback that's due you, you simply appear at the slot club booth and ask an attendant for it. You'll have to present photo ID. Then you'll be issued a voucher that you take to the casino cashier (cage). There, you'll again be asked for your valid identification, at which time the amount of cashback on your voucher from the slot club booth will be handed over. That's all there is to it.
