I realise there are probably much better ways of doing this exact same thing (especially with the full photoshop), but I like this method as if gives you a lot of control over the size/style/opacity of the shadow.
*If you click on any of the images, you can see a larger version*
1. First you will need a picture of a sim, preferably on a plain background (not necessary but makes life a bit easier). Then you will need to cut the sim out so that they are on a transparent background:
2. Select the transparent area around the sim any way that you want (I used the magic wand tool so that I could get it all in one click), making sure to select any bits on the inside that may be overlooked (such as the hole in her arm):
3.Now invert the selection so that you only have the sim selected:
4. Expand this by how many pixels away from the body you want the shadow to end (there can be some guesswork involved as it depends on how large the images is as to how much you will need to expand it by):
5. And now feather that by the same number of pixels as in the last step (this creates the blur effect):
6. Now create a new transparent layer under the sim:
7. And, with the same selection, but on the transparent layer, get the paint bucket tool, and fill the selection with the colour you want the shadow to be (black works best but other colours can create some nice effects)
8. You can then change the opacity (transparency) of the shadow if it is too dark, or you can duplicate the layer if it is not dark enough.
This is what the final result looks like:
Hope this is of some help to people
*By the way, in the final image, I'd also rotated her a bit, and added a warming photo filter, so that's why she looks slightly different from in the tutorial*