I came across this one at http://apofiss.deviantart.com/art/MAGIC-bubble-tutorial-172319836 which has helped me create my first bubble illustration. Although I didn't follow it step by step, it gave me a good introduction into where I should start.
Using tips from the tutorial, I started to try and create my own, playing about with the different Photoshop layer settings to give different effects. Here is my first experiment. I am hoping to create a few more before choosing a final image to use for the game.