1. Create a new Photoshop file (Ctrl+N or Cmd+N for the Mac) with "Background Contents" set to white. Click OK to accept your settings. I used the "640 x 480" preset and the numbers for this tutorial work at that size. If you want to make a larger image you may have to change the number significantly to make the effects look right in your image.
2. Press D to set your foreground and background colors to the default (black and white.)
3. Go to Filter>Render>Clouds like in the image below
and click OK to add a cloud texture to your image.
4. Go to Filter>Render>Difference Clouds (note in the image below that it is the menu choice under Clouds) and click OK to add a difference coulds filter effect to your image.
5. Hit Ctrl+F (Cmd+F on the Mac) to repeat the Difference Clouds filter. You are trying to get some nice lines - if you don't get them the first try, keep hitting Ctrl+F (Cmd+F on the Mac) until you like what you see. If you go too far and the image doesn't look good any more Ctrl+Z (Cmd+Z on the Mac) will undo your last step, but hitting Ctrl+Z again will redo your last command. To do multiple undo's in Photoshop press Ctrl+Alt+Z (Cmd+Opt+Z on the Mac.) If you go backward too far, Ctrl+Shift+Z (Cmd+Shift+Z on the Mac) will redo multiple times.
Keyboard shortcut review for this page (Ctrl = Cmd and Alt = Opt for Mac):
Ctrl+N = New Document
D = Default Color Swatches
Ctrl+F = Apply most recent filter with exactly the same settings
Ctrl+Z = Undo (toggle with Redo)
Ctrl+Alt+Z = Multiple Undo (go backward through the History)
Ctrl+Shift+Z = Multiple Redo (go forward through the History)