Take help of this example to learn how to increase nails. I have here a finger picture to increase the nail.
Open a Picture: Capture any picture on nails.
Selection: Choose Polygonal lasso tool (L key) and make selection as I have done here.
Take color: Choose Clone stamp tool (S key) and go to in the selection, press Alt key and click mouse on the nail to take color.
Scroll you mouse in the selection then press Ctr + D to hide your selection.
Final picture is here.
Recommend the tutorial