The most important thing when making a collage like this, is to find 2 images that have similar perspectives, so that the final image looks a little more realistic. I will be doing more detailed tutorials in the future, this is just a general set of tips and steps to follow:
1. Find images with similar perspectives (matching light is recomended too)
2. Overlay them, make sure the part where the images connect (floor in this case) matches well
3. Cut the subject and insert it to the background
4. Add firsts shadows
5. Match lights and colours (more tutotias for this soon)
6. Add finals shadows and smooth edges (optional)
7. Final effects to both the subject and bg, to make it look like only 1 image