Going from memory I believe I added a new layer. Filter > Render > Clouds. Can't remember if I spammed it two or 3 times, not on pc so can't double check.
Then Ctrl + L to edit the levels. You basically want a black background with white lines (just invert it afterwards if it is wrong way around) play until you get the effect you want.
Set it to screen (I think) lower the opacity if needed, and use a clipping mask to hide the excess you don't want/need.
hope this helps.
