Select the earth (using the Circular Marquee tool)
step 6 of 12
Cut and paste the photo of the Earth into the main document.
step 7 of 12
Add a shadow effect to the Earth (see the values in the picture).
step 8 of 12
Download a photo of the moon, and repeat the process of placing it in the photo (same as the process for placing the earth). See the link for the photo.