First, go to the gas station. You'll need to go inside the station to pay for gas with cash. Let the staff know how many dollars you want to pay in cash, and what pump number you'll be using. They'll ask you for the cash, and you can then pay it. You might be able to fill up with gas first and then pay inside. But you'll need to check on the pump's display as this will tell you which to do first. Lastly, fill up at the pump, and collect your receipt afterward.