The most important part of a painting is the drawing. If the drawing is not correct, no amount of value or color or details will make the painting successful. Because the drawing for this subject consists mostly of various structures, getting the perspective and placement of these elements is vital.
How do I choose a pencil? Pencils are made from graphite (a form of carbon) combined with clay. The more clay, the harder the pencil. Hard pencils are defined by the letter “H”. The higher the number, the harder the pencil. Soft pencils are defined by the letter "B". The softer the pencil the higher the number. 2B is soft, 3B is softer, etc.
Higher clay content in a pencil makes it more difficult to erase. You can use a light touch when drawing with a hard pencil, but the lines will disappear when you apply a wash over them. If you press down too hard so that the drawing is still visible, the pencil lines will create a groove in the paper that will fill with pigment when you paint over it, making the lines impossible to erase.
A softer pencil will work better. Keep in mind, if you use a pencil that is too soft, the lines can smear when you paint over them.
My personal choice is a 5B pencil. It is dark enough to be seen as I paint, yet soft enough to be erased.
Erasing itself is another issue in watercolor painting, and another reason to plan your preliminary drawing carefully. If you use an eraser that is abrasive, you will alter the surface of the paper, changing the texture of the paper and how the wash itself is absorbed.
The kneaded eraser is the least abrasive, and therefore my first choice. If that does not work, and a more abrasive eraser is necessary, I will use one only when the painting is finished, keeping in mind that the paint can be erased also.
Why erase pencil lines at all? There is a difference of opinion regarding drawings in watercolor and in all other medium. For some reason the "experts" have decided that the drawing in a watercolor painting can remain visible and become part of the finished painting. For all other medium this is not considered acceptable. Personally, I do not want any part of the drawing to interfere with my finished painting.
Remember Rule #42-The success of a painting occurs before you pick up your brush.
Next step-what colors should I use?