Read the instructions with the salt you puchase. It will give you an amount per gallon, sometimes cups, sometimes a weight. The salt needs to mix with the water, most people use a small powerhead or pump for this. Then you also need to get the temperature close to you tank, with a heater. Let the salt mix up for at least two hours, in an open container to gas off as well with the air. Its best to add the salt slowly the first time you make a batch, this way you can add the salt and bring it up to the level you desire, like, for example, 1.025. Let the water mix overnight, and you shoudld be good to go. Make sure you use a good device to measure your salinity, a hydrometer is not always accurate, you should use refractometer, and calibrate it as well. I hope I was clear and unbias...