Well, from what I've read, it's best to do it in a smaller container or tank. I use a tank that's like half a gallon, then fill 50% of it with the salt water I've made. To make it, you get aquarium salt, add one teaspoon to one gallon of treated water, mix in the salt until it's fully dissolved. The key part is to make sure the salt water is the same temp as your tank water so your fish wont go into shock.
Once you've done all that, let your fish sit in the water for 5 minutes, but watch him the whole time because you'll want to remove him if he goes unconscious, and then either put him back in his tank or in a seperate container with treated water that has no salt. If he does fine after 5 minutes, you'll want to add some of his tank water to the salt water so it's 2/3 tank water and 1/3 salt water, then let him hang out for 10-15 more minutes.