This is the original picture that started the post.  Which was, "How to I make it look like she is not floating in midair?"

First take your selection tool and select part of the bottom where you would imagine the floor would be.

I have it on "0" feathered edge.

Copy and paste that on to another layer. So now you have two layers.

I used "Levels" to darken my new floor selection.You can also use Curves, Midtone and Shadow, Brighntess Contrast. Whatever your favorite lighting tool is that will make the selection darker.

This step is optional.  I did it because the background has a slight draped look.  If it was a straight background or just a color then I wouldn't do it.

First I merged the layers into one. Then I took the "Warp" brush and and warped the line up and down close to where the drape folds were.

I wanted to keep the illusion that it was a continuation of the folds, although most of it will get lost there is will be just enough to help with the illusion.

Here is another step that you can adapt to your workflow.

We need the edge to be shadow as it would be in real life.  You can use the burn tool by free hand or if you want to be a little more accurate this will work for you.

I took the selection tool and as you can see on the menu I set the feathered edge to 63 and selected the floor line as seen here.

The reason I have such a high feathered edge is, when you run the burn tool, which was set at only 18 percent strength it will be a heavier burn in the middle and a go lighter towards the edge of  your selection because of the feather.

Now I tool the blur brush and I blured the line so there were no definitive lines left.

This is just tweaking the floor with the Dodge and Burn Tool.

I used the Dodge tool at around 18 percent strength and made the middle section lighter.  Took the burn tool and ran it aournd the edges and front.

I cut Carrie's daughter out of the original picture and placed her here on a separate layer.

Then I slected the backgound. On the right you can see the lighting element I added to the background only.

I added the shadow to her and as you can see it falls onto the background in a very believable manner.  

But our goal is not to have such an obvious shadow but to have just enough to give believeabiltiy that she was there but not enough to make it look like we didn't know what we were doing.

Here is the final shadow.  You can see the outline that shows where it is in the picture.

I adjusted the shadow just enough to be almost subliminal.  You know it's there but it isn't obtrusive.

About here I realized that I had her feet to close to the edge and decided to extend the bottom of the canvas.

I merged all the layers and then used the burn tool around the front of her shoes to add more anchor to the floor.

I extended the canvas and used the clone tool to fill in the gap.

Here's the end results. The one on the right is before adding the vignette which is on the left.

Keep in mind the goal which is to give the illusion of a floor.  I could do this tomorrow and it might be just a little bit different.  I could add more shadows or highlights, I might not blur the floor line as much, etc.  For example, when I look at this background I see a flow that I want to continue with the floor.  So I blurred it more then I would a solid colored background. Your personal taste is what will dictate the final look which you can only develop from practicing and then practicing again.