A good watercolor is based on a good drawing. What you usually need is a contour drawing of the kind that Charles Reid talks about. In fact, many “watercolors” are often just colored drawings. You can, of course, paint over the details in the drawing, but you have to have the perspective right, the gestures, and so forth, so you have to have a drawing with which to start. One way to get that drawing on the paper for a watercolor is to “square up” an image from somewhere else. I have a post on the details here. Then you can decide how much of the pencil marks to leave for the viewer to see. The problem in the field has been that you can’t get that image from which to work. You would have to carry both a camera and a portable printer.
That has all changed with the ubiquitous smart phone or tablet. The smart phone is always in your pocket and therefore always available, and the mini ipad is pretty portable. There are several apps for smartphones and tablets that do the squaring up of a photograph you have just taken. Here is an image from one I like, an app called Copyit:
There are grid lines (the black lines) whose number and thickness can be varied (I have just 6 boxes in the above image), and there are “guide lines” within the grids in red (16 within each grid in the above). If the grid happens to be obscuring something, you can move it. You can change the photo temporarily to black and white to get a value picture. You can zoom in with the usual two finger gesture to look more closely at a grid. You can save the result like that above to your photo album, print it out, etc. The developer has thought of most things. There is a help function and a manual that you can print out which is not particular written with a new-to-the-app point of view.
I hope it’s clear that you have to draw the grid on your paper and the “guide lines” as well in crucial areas.
There are a lot of people out there who are against using photographs in general. There is something to their criticism, but it applies to people who are trying to copy a photograph rather than use a photograph to make measurements on an already sketched image. You should definitely start with a freehand sketch of anything your doing. When you do that, you will notice that you are making measurements usually against some line on your drawing that you have decided to hold constant (like the length of the statue in the above photograph. The waist is about 1/3 of the way down that line, etc). This is really not much different from how you work with a grid. It’s just that the constant lines against which you are making measurements are the added grid lines. Using the thin red guide lines the waist is about 1/4 of the way down the red line in the box.
Unfortunately I usually can’t spend a whole day outside doing a painting. That means that my drawing has to be of a rather simple subject if I am to do it completely freehand. With this app I can take on much more challenging subjects and get a drawing down much quicker than before.
By the way, it is, in fact, very hard to make a drawing or a painting look like the light in a photograph, and most people can’t do it. So don’t worry. The problem is that most photographs are not particularly good, have too much detail, etc.
A search (e.g. Google) of the term Copyit will bring up several options to explore including a UTube video, the support page, and the app from iTunes. If you carry a smart phone you should consider Copyit.