I'm using nearly 2 gallons of water, 4 cups of kosher salt and 2 cups of sugar. This is a lot to mix. While it is very important to keep the turkey cold, I'm going to bring 4 cups of water to a near boil. I'm adding about one cup to the sugar, which dissolves easily and the rest of the hot water to the salt. Now water can only hold so much salt, but these three cups of hot water are going to get the process started and allow me to get that salt moving. It is vitally important to get the salt as completely dissolved into the water as possible. Any crystals leftover are going to float to the bottom of the container and just sit there.
Alternatively, you can combine all the ingredients for your brine in a large pot and bring the whole mixture to a near boil. However, you will need to cool the brine before placing the turkey in it. The brine needs to be cold, very cold to prevent the turkey from spoiling in the brine.