Now, here's the deal: Because this scene is a gross fabrication, we want to do as little as possible to tip our Photoshop hand and add something that screams “retouched!!!” Therefore, we won't add a ton of shadow to the new image, but instead a whisper or two.
Here's how to add a realistic hint of shading to the composition:
Adding Shadows to the Scene
Zoom out to 100% viewing resolution.
Click on the Create a new layer icon on the Layers palette. A new layer will appear between the Les Paul guitar and the Background layer. Rename this layer Guitar Shadow if you like.
Ctrl()+click on the Les Paul layer thumbnail ...