Adding a cast shadow can be a bit tricky at first but its not as difficult as it seems.
In the image below, you'll notice that one side of the shape is light and the other is darkened. All you need to know for now or keep in mind is just that. What the light doesn't hit, is darkened.
You also have to ask yourself where is the light source coming from and how far away is it?
It is important to keep in mind that shadows are crisper and darker if the light source is close to the object or lighter and more diffused the further away the light source is.
In the image below you can see a very slight cast shadow. This would indicate the light source is far away from the girl and rather diffused.
Whereas in the image below there is a very distinct cast/drop shadow
Here is a nifty trick to help you place your cast shadow.
Create a mask of your image using Mask It.
Determine your light source, and then place your mask in the correct position.
You can then draw around your mask and shade in your cast/drop shadow appropriately. It is important to note the shadow takes on the shape of the item it comes from..
Now you have a go!
** Please Note: All shadows in this post have been darkened considerably for clearer understanding of the concept!
Happy Cast Shadowing .... Mandi-Lee