Putting 3D into a 2D photo

When applying a 3D model into a 2D photo, there are a few things to consider in order to achieve a look that ensures the object appears to belong to the photograph:

For starters, knowing the height of objects in the world. By measuring things close to where the 3D model will stand, you can get an understanding of the size of your model and how it should be scaled in the world.

Taking reference photos of objects around where the model will stand (for instance a light post or a building) will give you an idea of where a fake object can be placed to mimic things like shadows.

Knowing where the lights are in the scene and which direction they come from is also helpful, as extra light may be needed in the scene to highlight the character/object more.

Environment ball/reflection capture is an accurate way of simulating the light/shadows/colour an environment and the surroundings reflect onto your model.

An example of an environment map.

Adding fake props/ground into the 3d world to use as dummies to capture light/shadow are also very useful. The object/character may be casting a shadow or refracting light onto the scene, but the photo does not understand that the shadow and light should be cast. So the shadow is baked into models and faked

 by turning the shadow into a 2D image that can be applied over the image.

Some of these effects can be faked by just creating the shadows/reflections and light in programmes like Photoshop and adjusted on an alpha channel. If the programme cannot do a particular task, creating your own effects on new layers may be necessary.