If your using the fizzy drink with a packet cake mix method
It is 100g mix to 60 ml of fizzy liquid for a mug cake and microwave for 90 seconds.
Or 500g mix to 300ml fizzy liquid and bake in oven.
The fizzy drink replaces an additional oil/egg.
Just pop powder mix in bowl, add fizzy liquid, mix and then bake.
Or why not make one in a frying pan in 5 mins!https://www.youtube.com/watch?v=PJL4sYWp5Q0&feature=youtu.be
You can make some great combinations as fizzy drinks come in such wonderful flavours.