How to make a realistic shadow from this picture?

Something I do is use the Smudge tool to reshape shadow layers pulling and pushing pixels into a better position.

You can blur just one part of a shadow layer by selecting it.  It is sometimes best to feather the selection when doing this.

A good way to assess how much feather is to use Quick Mask.

Enter quick Mask and apply Gaussian Blur to Quick Mask.

The preview will show how much feather is being applied

Come out of Quick Mask and apply Gaussian Blur.  I was only able to use a value of 5 pixels.  More than that, and the transition became apparent.  I could have stepped back and increased the feather though.

Reshaped with the Smudge tool.  This trick is a real time saver.

As with all illustration, if you are not sure how it should look, go find some real life photographs with similar lighting.

Hi

In addition to all these good answers, try to add a more concentrate shadow right under the body touching the floor

E

Dave's approach is the way to go here, and the key point to getting it realistic is what he says here:

Keep the shadows dark where the items are close to the ground and faded further away.

This is important. A real shadow isn't uniform, and that's why a click-the-button Drop Shadow doesn't look realistic.

Since the shadow already exists and is available in the Red Channel (R) of the red background Image, copy the R channel and add it as a Layer on the silhouetted version with a transparent background. Place a clear layer below the R layer and turn off the layer that has the color image on a transparent background.

The layer stack becomes R channel on top, clear layer in the middle, and the silhouetted color image on its transparent surround on bottom.

Go to the top layer, the copy of the R channel, double click to bring up Layer Style and in the Blend If section move the top right slider as shown. (To split the slider, hold down the Option key and click and drag the left side of the slider.) Then drag the bottom layer up so it becomes the top layer.

In the set shown below it is placed on a white background.

You have the shadow there on a solid background. I would just extract the shadow:

Hi

I'd forget the drop shadow. Just decide where your lights are coming from then paint using a soft round brush at opacity c25% and on a layer set to multiply blend mode. Keep the shadows dark where the items are close to the ground and faded further away. If you overdo it just switch the brush to white (keeping the brush opacity to c25%) and paint the overdone shadows out.

Dave