Create a new transparent layer above your first letter (and all of its effects). Name it (in my case) "s-texture." Open our wooden texture from earlier as a new layer. Right click the original S (or your corresponding letter) layer and hit "Alpha to Selection". Click the wooden texture and hit CTRL-I to invert the selection. Hit delete. Merge the wooden texture (now in the shape of our first letter) with the s-texture layer.