Once you have measured how much water you need, write it down. I can't tell you how many times I have forgotten the number when it came time to fill the container and had to start from scratch.
Now it is time to do the math. You need 1 cup of table salt per gallon of water to get a proper brine. This works out to 1 tablespoon of salt per cup of water (16 cups in a gallon, 16 tablespoons in a cup). Make sure you are not using salt with iodine. Iodine will spoil the flavor.
Not all salt is created equally (see my article on salt). If you are using kosher salt you need to add twice as much salt since it is larger in crystal and lighter by volume. If you want accuracy, weigh the salt. You want 10 ounces by weight of salt per gallon of water.
In my brine I need 30 cups of water or 30 tablespoons of salt (2 cups of table salt minus 2 tablespoons). I am using a kosher salt so I am using just a little short of 4 cups of salt.
While not necessary it is a good idea to add something sweet to your brine. Sugar will offset the salt flavor and really adds something to the turkey, so I always add sugar to my brine. You do not have to be as accurate with the sugar as you are with the salt. Add one cup of sugar per gallon. I am using my favorite Demerara Sugar which is a dark, cane sugar. Personally, I just like the flavor.