For Citibank Stepup, you need to add $ into your account, no matter even it's only $1. The interest rate will increase every month up to 12 month. If you withdraw more than what you deposit in a month, the interest rate will drop to the next tier. Flexible with ATM. But, the interest is SO LOW now around 0.3%, i don't think it worths the effort.
For SCB, just keep the amount for 1 year, the interest will be calculated and increased every 3 months. You are allowed to withdraw, but the interest rate amount that is started from the last calculation of interest rate will be forfeited. No partial withdrawal is allowed. There are another type of stepup that does not allow withdrawal till 2 years, higher interest.