I'll see if I can explain it. I'm using Photoshop7 so the things might be a little different depending on what you're using.

First you have to make that scratch file a pattern. Open the scratch picture. Under Edit go to Define Pattern. Then go to whatever picture you want to edit it on and make a New Layer (Shit+Control+N) and then go to Fill and change the fill to Pattern and select the scratch one. Then on your layer box change the place where it says Normal to Soft Light or just play with the various options till you find one you like.

I hope that's what you were asking for else I'm an idiot :)