How to Make a Big Fluffy Cloud

First you go to Deviant Art home page.
Then go to the Deviant Art Muro.
Then you go to the bottom right hand corner and choose to make your screen blue.
As you can see below. 

Step 1:

Then you are going to go near the left bottom corner and choose the paint brush 

Step 2:

Then you go to the bottom right side.
The tool to the left of the text is the tool you need.

Step 3:

Then take that tool and but the color wheel to any color and change that to white.

Step 4:

Lastly what you want to do is make the cloud. 
You can make it how big you want or how short. 
This is my final below.