I usually use Hibbary’s paper texture whenever I want that grainyish look. You can find them on dA!
I put the texture on a new layer OVER the layer that I want it to show upon. Then I just put it on overlay/multiply and select the clipping group effect, that makes the texture show up only on the area the lower layer has in it.
God, that must be the most awful explanation ever, I just woke up. Hope it helps. Thanks for praises, ahhh!