To start with I would add 1tbs per five gallons of water. Each time add one more tablespoon. It helps to keep a chart of how many tablespoons you are adding each time. Do this until you reach full saltwater
What I do is in a bucket add the salt, fill it up with hot water mix it up good and put an air stone in it and leave it over night. Then when I'm filling my tank up with the hose slowly add the salt water in the tank while filling the tank back up.
Always measure the salinity of the water after. I also measure the salinity in the bucket.