Start a new document about 800x600.

Fill the background with a blue gradient. (#395D9E -> #67ADF0)

- Create a new layer.

- Press the "D" key to reset your colors to black and white.

- Go to Filter->Render->Clouds

- Then go to Filter->Render-> Difference Clouds.

- Press CTRL+F twice to reapply the filter.

- Duplicate the clouds layer (CTRL+J) flip the copied layer horizontally (Edit->Transform->Flip Horizontally)

- Set the copied layer's blending mode to SCREEN

- Merge the 2 cloud layers together to become 1. (to do this, link the 2 clouds layer together and go to Layer->Merge Linked)

- Set the cloud layer blending mode to Screen

Now set your Foreground color to black if its not already black.

- Select the Brush tool and select brush size 100 SOFT EDGE.

- Now use the brush tool and draw your shape. Do this ON the cloud layer!.

- Now go to Image->Adjustment->Levels, and set the input levels to: 20, 1.00, 120. (or you can adjust the sliders til you see something you like).