Realistic Clouds - part 1

First step is to make a cloud shape using the color that is going to be the darkest color in the cloud. then use a custom brush to erase around the edges of the clouds shape.

We will use a guassian blur to soften the edges of the cloud shape.

Now select the dark blue and light blue of the original cloud and do a linear gradient.

Next we will select a custom brush to make some shapes. hit (Q) to enable mask, then brush in some area's, these area's will give depth to the cloud. hit (Q) again to make selection. to inverse selection go Ctrl Shift and (I) at the same time. use a custom brush for the new cloud shapes too. Ctrl + D to deselct.

We will use a custom brush to erase and "sharpen" the edges of the cloud slightly.