Here you go. The technique uses a clipping mask. Short explanation after image.
You can see my layers. I'll explain from the top down.
Make a layer of text.
Make a copy of the BG you want to use/have come through the letters. Move this layer above the text.
Turn off the original BG layer below.
Hold down the alt/opt key and click between the text layer and the layer above. This creates a clipping mask. In other words, clips out all the pixels around the letters, masking them (but not the same as a layer mask to prevent confusion). The pixels of the top layer are all still there so you can move that layer around all you want and reposition the image if you like.
Now move down to the original BG layer. In order to make the letters show, there are a couple things (at least) that you can do. Specifically lower the opacity of the layer. (make a white bottom layer so you can see the effects. Or for that matter, a colored layer if you want a different effect for the BG, but that's not what you're asking)
Another way to make the text stand out is what you see here, a bit of bevel layer fx.
The other thing I did was copy the BG layer, leave it at normal but with decreased opacity and change the layer mode of the one above to overlay. You can experiment with that if you like.
This is a basic overview of using the clipping layer with text. It's fun to use. I hope you enjoy it.
