1/2 cup per gallon of water should get you to 1.023, I think. If I'm mixing 5 gallons, I'll usually do 3 cups, then end up having to add more later. I'm sure there is a mathematical formula I could use to get me to 1.025, but I'm not that smart...
Water first, add heater, powerhead, and salt. Let it mix for a couple hours (some people say 24) and you should be good to go.