The beauty of the screenshot I found for this one (and I can't remember where I got it, alas) is that the background was already that lovely mottled blue-green. All I did was apply a layer of thin vertical stripes, with the opacity set to around 40% -- most people would do it with a brush, but since I have Photoshop Elements and most of the user-made icon brushes around are for a more advanced version of Photoshop, I just used the GIF image of the brush instead. Then I used the eraser tool on that layer to remove the stripes from Snape's face and body.

I found that in order for the text to show up properly against that brushed background, however, I also had to carefully erase around the text as well -- otherwise, even if I put the text layer on top of the brush layer, it turned out all jaggedy-looking.

Hope that answers your question!