There is tons of room for the imagination with this one. How to make the pants filled out is the easy part! In this set up my model was wearing a black turtleneck shirt and black socks with the blue denim stonewashed jeans. By using a black background and an old chair, an illusion of a pair of pants with not body is simple.
The lighting was simple. Two hotlights with umbrellas, one to the left of the subject and one to the right. A SB800 to the imediate left of the camera with a white umbrella for diffusion. First a picture of the chair alone was obtained for layering purposes. Then the model sits with his arms up over his head. It is important that you keep the focal length and chair in the exact same position for both shots.
The Nikon D300 was set on monochrome b/w and the iso 200. The SS was 1/125, F/10.0, FL 22mm. In pp the contrast up was bumped a tad and a clarifying filter was added. Also a black vinette was added to the model image. The contrast and clarifying filter were run on both images. Next the pant layer was copied and pasted on top of the chair layer. Using the eraser tool, in psp or a masking layer in ps, remove the unwanted body parts to reveal the chair in those parts. Be sure to erase completely to the edges that are not going to be removed. Merge the layers when done. Because its sometimes hard to see the differents between black pixel shades on some monitors...you might need to make a copy of the image and paste it as a new image...then on that new image LOWER the contrast to the point of seeing shaded whites. If there is a difference in your background pixels they will show up as a 'halo' around our image.
Using the paint brush with black....paint over these areas on your ORIGINAL image using the copy for reference only. You can do the copy and paste as new image again when you think you have it all to be sure. The reason it is done this way is because some editing programs will not copy your exif info when you copy and paste as new layer. It is important to keep that info with the image. You can delete the copied image when you don't need it anymore. Save the original under a new file name and you will still have the original originals of the jeans and of the chair while keeping your exif info.
Next, if you would like to accentuate your highlights on the folds of the jeans...then use the dodge brush on the highlights and burn brush on the shadows of the folds in the fabric. Do this to your tastes.
This standing image was done the exact way except instead of a chair...the second image was the back of the pants. Pretty much done the same way...only the back of the pants layer was on top and resized to fit after erasing the background.