Brackish Water Preparation

Here is what you will need:

Salt: You must use marine salt and not regular aquarium salt.
Water: Use your usual conditioned source water.
Hydrometer: To measure specific gravity (s.g.) – salinity. I find the simple plastic type with the swinging arm to be best. It must measure at least as low as 1.010 on the scale and the more detailed the measurements are on the hydrometer, the better.
Water holding device: Such as a large new bucket, garbage can, etc.
Heater: To heat water to desired temperature. You may find a submersible brand easier to manage. Using a heater is optional in some cases (see comments). If using a heater, you will also want a thermometer, to make sure your prep. water is the same temperature as the tank water.
Powerhead or simple filter: To agitate the water to ensure that the salt dissolves completely. Optional in some cases (see comments).

1.5-2 Tablespoons of marine satlt per gallon water should get between a 1.012-1.016 specific gravity, depending upon the manufacturer of the marine salt.

1/2 cup marine salt per gallon water should get a 1.020-1.023 specific gravity, depending upon the manufacturer of the marine salt.

For less finicky fish, such as many brackish species, it may suffice to simply pre-mix a concentrated quantity of salt in a bucket, stirring well with a plastic spoon, and add warmer water later to get to the desired temperature. Most or all marine fish and organisms will not tolerate this type of practice, and the heater and powerhead options should be utilized for them.
It is important to let the salt/water mixture stand for 12 hours or more, to ensure that all of the different salts, buffers, and various chemicals have had a chance to fully dissolve in the water.
After the salt is dissolved, be sure to check the s.g. to make sure that it is within the range that you are aiming for (brackish water: 1.012-1.016 s.g.; saltwater: fish 1.020-1.023 s.g., reef 1.023 to 1.026 s.g.).
Brackish fish are very tolerant of swings in the salinity of their tanks, so only an approximate s.g. is necessary for them. However, the s.g. for marine fish needs to be very accurate in order to keep these fish stress-free and healthy and never more than a .002 difference.