It is imperative that you use only sodium chloride (NaCL) that is 99% pure. This is commonly available at most pool stores or hardware store. Do not use rock salt, salt with more than 1% yellow prussiate of soda, salt with more than 1% anti-caking additives. DO NOT USE IODIZED SALT.
To add salt turn the swimming pool filter on and add salt directly into the pool. Use a brush to dissolve and disperse salt throughout swimming pool. A pool vacuum will serve the same purposes. Run the pump for at least 24 hours using the main drain for suction. If you don't have a main drain, use the pool vacuum. Do not add salt through your skimmer; add directly into the swimming pool and brush to dissolve.
The only way to remove salt is to drain water out of the pool and fill with fresh water.