This is what we'll be aiming for today:

final

First, grab a picture, ready it to be cg'ed, and fill it with flat colours.

flat colours

Hold ctrl and click on the layer of the part that you are going to cut. Grab your polygonal lasso tool and hold alt and click away the part that you don't want to lighten, so you should have an area that is where the light is coming from...

selectionarea

Next, grab your gradient tool, and put it on these settings:

changing the gradient settings

The first colour is the flat colour of the area, the second colour is black. Select the circle gradient type and stick it on screen.

Now gradient away! Start the gradient a fair bit away from the selection, cos you don't want it to be too light, or you can meddle around with the opacity setting.

lightened

Now select the area you want to darken...

selection area 2

Set the gradient tool to these settings:

darken settings

Again, the first colour is the flat colour, the second colour this time is white, and we need it on the multiply setting.
Again, gradient-away! After doing a few more areas you should have something like this:

final