Hi STEVIE00
What are you treating your fish for?
Salt baths are an old remedy and still a good one, but I recommend using them with, not instead of newer medicines for the particular ailment they are suffering from.
I use 1 teaspoon of salt to 2 cups of tank water. Be sure that the salt is completely dissolved before adding the fish. It might take stirring for a little while or pouring back and forth between two containers.
Add the fish and watch him closely. At the beginning he might not be able to tolerate more than a few minutes in the bath. You can leave him in for 20 minutes or so before returning him to his tank, which should have medicated water. If he faints and rolls over (which is quite apt to happen at the start of the treatments) do not be alarmed. Simply net him out and return him to his tank where he will quickly revive.
You can repeat this treatment up to 6 times a day. If you are treating for an advanced stage of bacterial infection, you will see the difference after a very few treatments. The salt eats away at the infected tissue much the way peroxide cleans a human wound. The medicated water will help him heal.
This is an excellent treatment, but it is harsh and it's scary to see the fish pass out. I use it only as a last resort when other methods fail.