Do you have a mixing container? A mixing container is really the best way to do it. Alot of folks here (including myself) like to use something like a Brute trash can. Something that you can mix up 20% worth of water in and let it mix for about 24 hours before you add it to the tank.
However for now, since you don't have anything in there yet...you can probably just take out 20% worth and then mix up some 5g buckets and dump them in. I'd highly recommend getting yourself a mixing container for future use once you've added fish and inverts to your tank, and ESPECIALLY once you've added corals. This way you can mix the water, warm it up to the tank temp with a heater and give the salt proper time for the chemical changes to take place before you add it to the tank. Salt mixes best in cool water, so I let it mix for about a day and then warm it up to tank temp.
My mixing container, I think I've showed you before...a trash can (never been used for anything but this purpose ofcorse), a powerhead for mixing with a hose connected to it which also allows me to pump the water from the can to the tank.