Examples:
http://www.makehumancommunity.org/forum/download/file.php?id=5407&mode=view
or
http://www.makehumancommunity.org/forum/download/file.php?id=5424&mode=view
Normally you have to do the following steps:
- Find an appropriate background image.
- Find a seamless image for ground, or create one using a graphic tool.
- Manipulate background image, so that it is even at the bottom.
- In Blender: load background image as plane. Use emission shader. The example picture of Wonder Woman is made with a factor of 0.4 plus an additional orange color for the sunset, the other one with 1.0. Without emission shader, you will see shadows on the background.
- Create a plane for the ground.
- Do the setup like I explained in the first picture below.
- With hue/saturation you can change your ground to fit the background (It is the same ground I used in both examples ...)
- place character, camera, sun, lights.
- adjust background (size it or move it up or down)
That's it. Look if the perspective is working correctly.
You can also use cheap dirty tricks when you still have problems to combine background and ground and use a natural border element, like a wall. They are doing that even for professional movies ... so don't be ashame
(I started like this in my beginners pictures .... http://www.makehumancommunity.org/forum/download/file.php?id=3628&mode=view)