6. Once you have some lines you like, hit Ctrl+I (Cmd+I on the Mac) to inverse your colors - your prominent lines should be white like in the image below.
7. Go to Image>Adjustments>Levels to bring out more of the detail - be careful of creating solid white or black areas as they will not look good in the final outcome, but make sure your settings accentuate the lines a little like in the image below. To manipulate the levels move the black, white, and/or gray triangles along the line. To see your levels adjustments before clicking OK, make sure the Preview box is checked. Once your image looks the way you want, click OK. My settings were 59,1.47,255 but since the clouds are a random seed, your numbers may be a little different.
Keyboard shortcut review for this page (Ctrl = Cmd and Alt = Opt for Mac):
Ctrl+I = Invert Colors