Some of my more unusual requests have been to take facial features from one person and place them on another shot of the same person because the body position might be better in one shot than another. Other requests have been to take objects shot in an indoor photo studio and make them appear as if they have been shot outdoors in a different scene altogether. You must make sure any images brought into another image take on the characteristics of that image.
At some point in the process of composition, you'll have to create a crop mask for an image. Masking is typically done by creating a selection with the Pen tool, the Lasso tool, or by using the mask mode to paint in a mask. Any of these methods will allow you to make a selection of a particular part of an image. Cropping or making a selection of part of an image allows you to select a picture element to be placed on a different background, restrict a correction or an effect, or select many images for a montage or composition of many images. Selections must be done accurately and correctly to be effective. Again, the idea in retouching is to make images appear as though they have not been retouched at all.