Step 1: Place the picture you are mixing in place on top of the other picture. Change the size and shape to your liking using Ctrl T then select enter.

Step 2: Change the opacity to around 50% so you can see both photos evenly and then select the eraser tool, now you can start rubbing out the bits of the top layer you don’t want, in this case i have rubbed off the bits that didn’t fit on the building.

Step 3: Once the shaping is done that is pretty much finished, but in my case there was still a bit of blank space on the building so i decided to fill it in by placing the same photo over it again but the other way round so it fit perfectly into the blank space 

Step 4: Finally just do the same thing with the opacity and the eraser to the new layer and it should look something along the lines of the finished photo below