Need help with marine salt -setting up a new tank-

[snow1]

I'm buying the tank used. It comes with live rock but NO fish. I'm planning to add some fish in on aug 6th when I get back from my trip. The tank is fully running right now.

What I'm trying to ask has nothing to do with the live stock.
I'm just unsure on how or what kind of way I should add the new salt to the tank when I'm setting up the tank. Keep in mind that I can't mix ahead of time. Any help would be awsome as I'm going to pick it up in a few hours....