1. Start with a new canvas, 320px by 240px works best for this.
2. Select the gradient tool (G) and use the following settings:
3. Randomly draw out gradients mainly in a verticle fashion and make sure that you do not go too over the top. The below shows how to make it look it's best.
4. Go to "Filter > Stylise > Find Edges" and you should have something like below.
5. Create a new layer and fill it with white. Then go to the layer selection box and change the white layer to "Difference".
6. Voilla, your pic is done... It should look something similar to this.
