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It all depends on what images you are using. If the foreground (figure) has been shot with the specific intention of laying over the background, then it is relatively easy. You will need to separately match: edges, color and grain.

Edges: use a layer mask on the FG and paint your unwanted material. Edges should not be too sharp or blurry. If you can reference something in the BG then this will help.

Color: break this down into lightness, hue and saturation. Your tools will be curves, hue saturation and maybe levels. 

Grain: add or remove grain using the grain tool.

You will also need to add a cast shadow into the new environment. For this you will need to make a new layer, set it to Multiply blend mode and paint the shadow in. Again try to refer to shadows in your images that are already there.

I attach a pdf that covers the basics.