After doubling your money on a slot machine, the next step in the build-up method is to move up to a higher denomination machine. For example, if you start with a 25-cent machine and double your money, you would then find a $1 machine to play single credit bets. It's important to set aside your initial investment and some profits to ensure you don't lose everything. Continue this process, moving up through $5 and $10 machines, always playing single credits and setting aside money as you go. This gradual approach aims to increase your potential winnings while managing risk.