My first step is to first measure out a reasonably accurate perspective drawing in pencil. I cover the surface with a light ghost wash.
Then I place the watercolor tones with a flat brush. I start with the idea of doing a sepia sketch, but decide to bring in a cerulean/light red combination.
As time runs out, I switch to a brown fountain pen to describe the small, fine linear accents.
Since the fountain pen is water soluble, I can't add further watercolor washes without dissolving the lines. The reason for adding the penwork at the end of the process is that I was more interested in using the pen for selective small accents rather than for boundary lines.
Homemade sketchbook pochade easel using adjustable torque hinges