In movies the best special effects are the ones you can't detect. In photomontage applies the same.
How many times we look at a montage and instinctively we detect there something wrong; the subject clearly looks superimposed.
There is a very simple rule that applies in every image and it can make your subjects really appear like they were originally in the shot.
Rule of Horizon
1. The Horizon is always at the same level as the viewer's eye.
It doesn't matter if you are sitting, or standing, when you look straight ahead you see the horizon.
2. Small people like children will be below the line, bigger people above the line
3. You can change the emphasis of the picture for example placing a subject above the line making it appear we look up to them
With this simple rule now you can correctly position and scale different subjects in a composition
Here is a practical example.
Step 1 Set the Horizon
In your background image draw z-axis lines to show the image perspective
The point where all the z-lines cross is the vanishing point. Place a horizontal guide in that point a draw a line
This is the horizon
Step 2 Place your subjects
For this example we will use the same subject so you can better appreciate the effect.
Place you subject and scale it. It doesn't matter how small it is. Now place it so its eyes match the horizon line
Scale the same subject a little bigger, as before place it matching her eyes with the horizon line
We can do this several times and if we place our subjects along with the horizon line they will look correctly position and scale according to the room or background.
