I don't think you need any tutorials, it's pretty simple: you just have to put the clip you want to use as overlay on top of the other clip. then you set the upper video track on "lighten" (use the "Compositing mode" button to your left).

and for the scratchy effect you have to use textures. just google "grunge textures" or smth like that, and there are plenty of LJ communities like texturize.