Create a new image 400 x 300 (W x H)

Step 2.
Set Foreground to White and Background to Black

Step 3.
Filter>Render>Clouds

Step 4.
Filter>Render>Difference Clouds

Step 5.
Image>Adjustments>Invert (Ctrl+i)

Step 6.
Image>Adjustments>Levels (Ctrl+L)

Step 7.
Image>Adjustments>Color balance (Ctrl+B)
Midtones:

Shadows:

Highlights:

Step 7.
ENJOY