I drew out the scene using watercolor pencils (that way the lines disappear once they get wet.) Then, I went to work on what would eventually be the lightest parts of the painting. This process is done completely wet into wet, and I love to drop in pure colors so that the water can mix them for me. I was very careful to avoid the few areas that were to remain white (as you may know, white in watercolor comes from the paper, so don't lose it!)
The hardest part here is to paint dark enough. The areas that look so dark now will look so much lighter once I surround them with dark colors. When you're first starting out, it's probably better to err on the side of caution and be a little too light. Just don't be afraid to go back in later to darken things if the values (lights and darks) need to be adjusted.
Again, here I'm painting the areas that are hit with the lights of the city. I'm more used to painting midday scenes, so it's amazing how dark it felt like I was painting some of these areas. I just had to have faith that the buildings would look "lit up" once I put in the shadows and night sky.