It's actually pretty simple, though it can be pretty difficult at times, what I do is I think what kinda color I want my background to be, so with iron man one I wanted a light kinda cloudy color so I smudged the iron man picture and then I think I may have made a new layer smudged him again then used the image and blended him in by smudging the outlines of him slightly and so on 🙂 Hopefully that explains it and you haven't jumped out your bedroom window screaming 'I DON'T UNDERSTAND!! FOR THE LOVE OF GOD I DON'T UNDERSTAND!!!'