You need aquarium or kosher salt. Treat 1tsp per gallon for 10 days. Raise the temperature to 80*F and do daily water changes (100% changes being best). Remember when you do water changes you are removing some of the salt so always make sure you put some back in. For instance if you have a 10 gallon tank with 10 teaspoons of salt in it and you do a 50% water change you remove 5tsp of salt so you need to add 5 tsp back.
After the 10 days remove all the salt by doing water changes but don't add the salt back. Keep doing water changes and feed the highest quality foods you can get your hands on, frozen or live is most preferable. Keep up this routine and you'll begin to see regrowth.
I've had the best success by putting the fish in a small 1 or 2 gallon bare bottom tank with just the heater, salt, IAL (not necessary but beneficial), and a hide of some sort (plants or a cave). The small size makes it easy to do 100% water changes every day.
CLEAN CLEAN CLEAN water is the key. Honestly if you keep the water clean you don't need to use the salt at all.
To prevent fin rot keep the water clean, feed high quality foods, don't use things that can tear the fins such as plastic plants or sharp ornaments.