It’s time to choose the Pen Tool (P) and represent a kind of train under the hearts.



We’ll represent the train behind the third heart too.



We need repeating the same operation, but this time with the fourth heart.



The Gradient Overlay option from bellow needs to be applied for the second heart’s train:

 

 



Next Gradient Overlay option will be applied for the train belonging to the third heart:

 

 



Finally we have to apply the next shown parameters for the train that is situated behind the fourth heart.

 

 



Next stage includes drawing one more heart that should be placed behind the main big heart, drawn at the beginning.