I will usually dissolve it first, but I have added it to the tank without dissolving it. Just put a bit in at a time 1/4 tsp, let it dissolve, add the next 1/4.
I think you are to use like 1tbsp per 5 gallons (1/2 tsp per gallon) for a freshwater tank (brackish tanks will be different, of course). There are directions on the box. Use aquarium salt only unless specifically using epson salt for medication.
People will argue either side of the salt issue. Do some reading, listen to people in here, and make up your own mind.