In general, it's best to pick the machines with the highest payouts. Search online to find specific machines that are known to have a high RTP ( return to player ). Pay attention to volatility, too. Low volatility slots have a higher chance of winning, but high volatility machines usually offer a bigger payout.
Show your ID to the casino staff, so they can verify your identity and age. Fill out a tax form, if you are in the US or a US citizen, so the casino can report your winnings to the IRS. Choose how you want to get paid, either in cash, check, or wire transfer.