From the "help changing background thread"

[Shannon1213]

On the maternity image with the grass replacing the sidewalk, is changing the blending mode to multiply on the new layer the way you kept the original shadows that were on the sidewalk on the new grass? Or did you carefully clone those in? Thank you! 

[Christina Keddie]

I'm just going to go ahead and link the thread, for anyone else following along at home.  

[Damien Symonds]

Yes, the blend mode is the key.  Multiply worked because the sidewalk was very light in colour.  If it had been darker, another blend mode might have worked better, or none at all.  That's why I linked this article in the instructions.