Yes you are only gonna use the dry cake mix, do not add anything as instructed on the cake mix box. Choose a diet soda similar in color of the cake mix used. For example: white cake, use ginger ale or a lemon lime diet soda; chocolate cake, use a dark colored soda such as root beer, etc.
Preheat oven per package instructions for the type of cake pan you are using. Wait until oven is preheated before mixing. When oven is hot, prepare your cake pan with whatever non-stick product of your choosing. Put dry cake mix in bowl first, nothing else. Add a can of appropriate colored diet soda and mix with mixer immediately until well blended. Pour immediately into cake pan(s), and place in oven. Cook per package instructions, keeping in mind each oven is a little bit different. Check cake about 5 minutes before time is up, using a knife in the center. You will need to add more time is center is still uncooked. The batter should be sticky on the knife, not batter liquid in the center. This cake will be mega moist, and each time I make it I get asked (and then that suprised look) when I tell them what it is. You may need to keep adding time in 5 minute increments until cake is done. Simply make a note for your oven's cooking time for the next time you make the recipe. Note: Cake will not rise as high as normal, that is why it is mega moist. Frost if desired. Cupcakes do well also. Note: You may use decaffinated soda. Regular soda's are okay to use if you don't mind all that extra sugar and calories.
If you a Weight Watcher's member, simply use the nutritional information on the cake mix listed in the dry column, plus nutritional info for the soda, and of course the number of servings. Don't forget about the icing if you add that to the cake.