With the sugar dissolved and the salt almost dissolved it is time to start putting together the brine. Now I have lined my container with a large zip-top bag. Keeping the salt mixture moving, pour it into the container and immediately start adding water (some of which can take the form of ice to keep the brine cool). This way you will get the salt completely dissolved. Once you have all the water in add your sugar/water mixture and whatever additional flavorings you are using.
Stir continuously scrapping along the bottom. If there is salt down there you should be able to feel it. Continue mixing until there is no salt crystals left. Only then can you add the turkey.
One hint: If you make the brine in advance the flavors you add will blend more.