Then using PhotoShop I add the proposed sign to the photo. Notice those black triangles in the corners? I put those on the artwork in my vector based program before I imported it to PhotoShop. They help with size and perspective. Above you see how it looks before adjusting for perspective. Below I have started the process by rotating it a little...
and now I have adjusted the perspective using Transform>Distort. I simply adjusted the corners to match the lines on the building while holding down the Control key.
Now all I have to do is erase the triangles and we can see how the sign will look.
But I like to make my photo mock-ups to look more realistic, so I have added the edge of the letters and a shadow too. On the shadow layer I decreased the opacity so any building details would still show; like that crack you see below.
So below is the finished photo. Looks pretty real, doesn't it? But il est faux! Nonetheless, it sold the job, so I'm happy.
Actually, I added the dimensions on the finished photo too because the customer has to get landlord approval and they will want to know not only what the sign will look like, but the size of it.