You can pay with a previous payout ticket, your member card, or cash. Often (if not always), the previous payout ticket will go in the same slot the cash can be inserted. Most machines take $5, $10, $20, $50, $100 dollar bills, though some also take $1, too. These areas will be lit up and unmissable.
The truth is that there is no trick to slot machines. They provide random results based on set mechanics and it all comes down to luck. With that being said, not all games are the same, so picking the right options is key, and you can still change the size of the bet throughout the session for better results.