I talked about the benefits of brackish water in the Brackish Vs Fresh Water Debate section, please read that section before proceeding.
Here is how you can make brackish water. First you need to purchase a Full Range Specific Gravity Meter at your local pet store, make sure it measures from 1.000 to 1.030. Second you will need rock salt, which can be purchased at the supermarket a few bucks for five pounds. If you are making a lot of brackish water you can use rock salt made for water softeners. Make sure the salt is at least 97% pure, it is normally only $5 for thirty pounds.
I use to keep my adult diamondback terrapins at 1.014. To obtain this level you should start out by adding 1.5 cups of rock salt for every 10 gallons of water. Let it circulate in the tank for 24 hours then measure the water with a specific gravity meter. It will probably be around 1.010. Add another half a cup of rock salt for every 10 gallons of water and wait another 24 hours before testing it again. Do this until the water reaches the saline level you desire. It normally takes about 3 cups of rock salt for every 10 gallons to obtain 1.018 salinity.
If you keep them in brackish water they will need to be soaked in fresh water for 15 minutes once or twice a week. Even though they can be house in brackish water they do need the access to fresh water.