Salt is good for healing open wounds, killing bacteria and parasites like ich, and promoting slime coatings which protect scales and fins, and makes colours more vibrant.
For the solution, it's best to follow the directions on your salt packaging, but generally it's half a teaspoon per gallon. Two gallons would be a whole teaspoon. Do you have a designated container in which you make batches of water for your water changes? If so, measure how much liquid it holds, and add salt accordingly. For example:
I used a 2L (Half gallon) gingerale pop bottle to make batches of water for the water changes in my half gallon tank. When I mixed the water, I would put 1/4 tsp. of salt in the gingerale bottle, along with water softener and chlorine remover. This automatically made the proportions of salt that I add to my water safe.
Salt is best used inconsistently. If you put the same amount of salt in ever batch, then the fish and the parasites in the water will become immune.
You can vary the salt concentration any way you want. I personally only add salt every other batch
(Not necessarily every other water change).
This way, there is always salt in the water, but the amount varies and it never becomes too concentrated.