In this tutorial I will show you an easy way to get some nice smooth looking bubbles like the image below. This tutorial is more of an outline, and cannot guarantee the same effect as below. You will have to learn how to use the dodge tool first.

Start off by making a new layer with the same color background, or similar as below.

Take out your circle selection tool and hold shift and drag to make a perfect circle selection.

Now get out your dodge tool and test it out. Make sure that you change the settings at the top to "highlights: Start off by adding some light at the top left as shown below.

Now with your circle still selection use your dodge tool to border around your bubble. You will see that your outer edges will grow light, while your top left corner grows extremely bright.

Now just repeat the process by adding a smaller bubble in the center.

You should now have an image like the one below.

Now lets keep repeating the effect with different sizes.

Step by step, making sure to first add light to the top and then border.

Now just add some smaller bubbles to make it look a little more realistic

To make it look like the bubbles are rising to the surface I used the dodge tool to add light coming from the left top corner as shown below.

Now you might want to add some italics to set the soft mood and you are done :)