Merging two photos isn’t easy.  You simply take one and align it with another.  Although in doing so, you have to make sure the perspective of the graphics line up.  Often times that can be the most challenging, along with the light.  One photo needs to be changed or color graded to match the other, or it can look out of place.  Sometimes dozens of actions in photoshop are used to get the final result. To truly get great results, you need a very clear, in focus picture to work with.  If you have that element, then often times the rest can be achieved.  If your picture is somewhat out of focus, no matter how hard you try, that focus is never there.  But, the beauty is combining different elements is you can take an average picture and turn it into something brilliant.