I've never done a treatment in a larger tank because I had been told to do large water changes everyday. Here's a link to what I use to follow: http://bettasplendid.weebly.com/fin-rot-101.html
You can also use tupperware containers or any plastic ware containers as well, but ONLY if they have not seen any sort of soap as soap is toxic to fish. So if you have some new ones laying around that would work, you'd just have to measure out the right amount of salt and dissolve it before you put it in the tank :)
But if you don't, you can put in 5.5 teaspoons of salt (make sure you dissolve it first, otherwise it burns) and then change out half the water everyday until the rot goes away. It discourages any bacteria growth when doing water changes. And then only put more salt in as much as you took out. So if you took out about 2.5 gallons of water, you can put it 2.5 teaspoons of salt since it doesn't evaporate.
But again, if I could have a more clear picture that'd be better because you might not even have to go through all of that!