Cartoon clouds

Step 1:

To get the right feeling for our clouds I will start by setting a sky blue color as my background, I ended up using #bedfe4 in my document.
In this step we will start by drawing the base for our cloud, select The Brush tool and open the brush preset picker. Set the Diameter to 40px and the Hardness to 70%, this will give soft smooth clouds.

Set the foreground color to #f3f3f3 and draw something that looks like the image below.

Step 2:

Now create a new layer with ctrl + shift + n, set the foreground color to #ededed, and change the Diameter of the brush to 20px.
Now draw with this new color inside the white so that the white is only visible in the edges of the cloud.
Vary the distance to the edge to enhance the cartoon feeling.
Bellow you can see how my cloud looked at this stage.

Step 3:

Now create a new layer with ctrl + shift + n, set the foreground color to #e9e9e9.
This step is the same as the previous except now you are supposed to draw inside the light grey color.

The result

Bellow is my finished cloud, you can improve the look of the cloud by increasing the hardness of the brush, but this makes it harder to get smooth edges so you have to be steady on your hand.
If you use smaller brushes you can add more details to the clouds and make them look more unique.