The item you wish to shadow should be in its own layer. To begin with, we will be making a drop shadow for a bow.

Create a new layer under the layer with the bow.

Holding down the control key, click on the tiny image, the thumbnail image, of the bow in its layer. This will select the bow.

Select a gray color for your foreground, select the shadow layer and then pour the gray color into the bow selection. You will end up with a gray bow in the shadow layer. If you turn off the visibility of the green bow layer, you will see it.

Turn the visibility of the green bow back on.

Making sure the layer with the gray bow is selected and that the image of the gray bow is NOT selected on your paper, go to Filter>Blur>Gaussian. Choose a 3.0 pixel radius.

You will see a slight gray edge around the green bow.

Set the blending mode of the gray bow layer to Color Burn.

Now you will see a slight shadow around the whole bow. This is what I call a depth shadow and it indicates that the bow is sitting on top of the page and is not an image that is part of the page background.

To get a drop shadow instead of a depth shadow, select the layer the shadow is on, make sure your move tool is selected and then use the arrows on your key board to nudge the shadow in the direction opposite of the light source. My light source is generally from the top left, so I nudge once down and once to the right and I get the correct drop shadow.

You can give your drop shadow a little interest by giving it some texture through the layer styles. Bring up the styles for the shadow layer and go to pattern overlay. Choose the heavy weave pattern with a hard light blend mode and your shadows will take on a texture and look a little more real.

Sometimes, we need a little more than just a plain shadow. The object may be at an angle, or may be higher off the page than norma. A good example is this safety pin.

Right now it just looks like an incomplete safety pin is laying flat on the page.

First make the shadow following the steps above.

Then go to Edit>Transform>Rotate to rotate the shadow around. Rotate it so the shadow meets the piece of the safety pin that is touching the paper where it goes through the paper.

Lower the opacity of the shadow layer down to about 40% and you have a great shadow. In this example the light source is coming from below. You will have to rotate the shadow in a different direction depending upon where your light source is.

To make your shadow even more realistic, go back to Filter>Blur>Gaussian Blur and make the shadow even more blurred. You can also go to Edit>Transform and rotate, perspective, distort and more to make the shadow dance to your own tune.

Here is another safety pin shadow that I distorted, blurred and played with. The light source for this image is coming from on top and a little to the right.