Alrighty, First off, create an image of whatever dimensions you want and fill the page with black.

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

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).

Set the layer to "Soft light" on the layers dialogue, which will leave you with something like this:

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!

Hopefully this is useful to someone besides me.