Could someone explain this to me more in-depth please:
Take the magic wand tool, click on the background, now go to Select->Invert.
Now cut the render out.
Now, switch back to your signature. Go to Layer->New Layer, and then paste your render into it. If the render is too big or facing the wrong way you can correct this by going to Layer->"Scale Layer" and Layer->Transform->"Flip Horizontally".
Now once you have it where you want it, anchor the layer by clicking outside of the signature.