The last important step to make your foreground object look like it belongs in the background picture is to add shadows. This can be done using your photo editors brightness tool. If you give the brightness tool a negative value, it becomes a darkness tool. This is where you will need your hand drawing talents. Up to this point all we did is trace.
Shadows basically have to be painted in.
The darkness tool makes the colors darker rather than completely covering over the image. Remember the angle of the sun, this will determine where the shadows need to be painted. Also if your foreground object is supposed to be touching something in the background, shadows are the best way to create this illusion, even if the shadows are not mathematically perfect. This is where the computer ends and artistic skills take over. Even after all the steps are done, there may be some things you have to do to make your picture look good.