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You will need a copy of Paint Shop Pro. I'm using vX but you should be able to adapt this to any release of PSP.
This tutorial illustrates how to apply shadows to objects outdoors.
Start with an new image. Make it size of your intended fnished product.
Apply your sky background, grass, building, tree, etc., in single layers.
Duplicate your tree layer 
and bring it to the top of your layers.
Activate this layer.
Use the Adjust Brightness/Contrast tool 
with the following settings:
Select the rectangle tool 
and pull out on side and top nodes to stretch it. Rotate it a little so that if covers as much of the roof (or the object you are shading)as you would like.
Turn down the opacity on the layer
Now you should have something like this
Now to remove the shading from the sky and tree. Activate your house layer (or whatever object you are shading.)
Choose the Selection tool and then Float 
Now choose the Invert tool 
and then activate the tree shadow layer.
Now hit the Delete key. Deselect.
Now you should have something like this
I used a simple object here to illustrate how this effect works. The examples below show the before and after with the shading applied to a few objects and the grass.
Have fun!
