Alrighty, First off, create an image of whatever dimensions you want and fill the page with black.
![Image](http://i.imgur.com/HrVr1.jpg)
Next, create a new layer and decide what colour you want the clouds to be. I'll do red (#FFFFFF) for this tutorial. Fill the second layer with this colour and set its opacity down to about 7.4
![Image](http://i.imgur.com/FfQMR.jpg)
Create a new layer and use filters -> render -> clouds -> solid noise. The critical options here are randomize and turbulent. Having a higher detail presumably makes sharper clouds (I haven't fiddled with this yet).
![Image](http://i.imgur.com/DTHRX.jpg)
Set the layer to "Soft light" on the layers dialogue, which will leave you with something like this:
![Image](http://i.imgur.com/oxYNt.jpg)
Create another layer and generate some more clouds, Ctrl-F should be sufficient here if you ticked Randomize. Set the layer to "Divide" and there you go!
![Image](http://i.imgur.com/weU4p.jpg)
Hopefully this is useful to someone besides me.