For the under drawing I begin on inexpensive trace paper rather then the final watercolor paper. The trace paper allows me to easily make corrections and make necessary overlay sketches. Once the under drawing is complete I will transfer it to watercolor paper.
Depending on the size of the structure I typically make my drawings 1/4" = 1'-0". For larger scale buildings I may reduce it to 1/8" scale. If it is possible to visit the structures I will measure as many details as possible, coupled with detail photos that I will organize back in the studio. Unfortunately in most cases I am unable to visit the buildings in person so I must rely other similar architectural details that I have previously measured, architectural reference books, and also my instinct of scale and proportion.
I always begin by laying out my center lines and overall proportions. Once I am pleased with the proportions I will then commit to drawing in the details. You don't want to spend a large amount of time on details and then realize that your proportions are all off. I have made the mistake many times so trust me when I say that it is frustrating to have to erase all of your work. This is also why it is best to start on trace paper. You want to avoid erasing as much as possible with your final watercolor paper. I will discuss that more in the further steps of the process.
For the tracing paper and the final watercolor paper I like to use a 0.5 lead holder, which keeps your lines fine and precise. The lead weight that I typically use is H or HB. You can use a little softer lead like HB for the tracing paper since you need it to transfer easily to the watercolor paper. For the watercolor paper I will use a harder lead because you want the lines to be very light for the final painting.