Getting hold of a good photo reference is normally only half the battle, after that you have to work out how to replicate it on your painting surface!
This is the reference image I used for this demonstration, a photo I took of the
Bombay Gin Distillery near Basingstoke. As you can see this is a complex reference image that contains perspective and buildings of different sizes, plus reflections, etc. Getting a good representation of this reference image is important and the process described below should help you with that.
I have a free image editor called "GIMP" that works very well to help with the sketching process.
GIMP is a very powerful image editor, some even say that it rivals Photoshop in what it can achieve, however the big difference is of course that GIMP is FREE to use. I have described below a sequence of actions that tell you where to go within the GIMP menu system and what to select at each stage. You should end up with an outlined image at the end as shown at the bottom of this article. Once you have your outline printed out then it is straightforward to put it beneath your paper and lightly trace the important lines to get perspective, relative size and alignment of key objects correctly.
You can make this transfer process even easier using tracedown paper or a light box or if you have neither of those things then make use of a window in the house! If you tape your printed image to a window the light from outside with act as your light box enabling you to be able to see the printed image beneath your watercolour paper.
The advantage of lightly tracing your image in this way is that it saves all of the rubbing out and trial and error associated with free hand sketching and this in turn saves disturbing your painting surface as little as possible.