The specifics of the watercolors is transparency, so white pigment is not widely used in traditional school techniques. This determines the way the shapes are painted with respect to the highlights and shadows, as well as requires the underlying sketch to be as subtle as possible (unless it is intended to be an active part of the finished composition).
Step 1: Working on a sketch.
First, while preparing the sketch, make sure that well-sharpened pencil is used. Pencil hardness of H or 2H might be optimal to produce faint and accurate lines without scratching the paper. The goal of the preliminary sketch is to ensure the pieces of the composition are properly and logically arranged with respect to the edges of the sheet and each other. No detailing is necessary at this step. Next, when the proper locations for the objects are determined and the compositional center is allocated, the study of the individual shapes starts. This is the art of lines and ellipses. Depending on the type of composition (nature landscape, architecture and city views, still life, portraits), the type of the refinement will be different, however, for each object some shape breakdown will be required. The goal of the detailed shape study is to gain a feel for the volume of the objects to be able to recreate it in 2D using tone and color work. The advice here is to draw the contours along the curvature (as if you were to wrap the wire around it), create a mesh that stretches onto the object. When the shape itself is prepared, the highlights and shadows are to appear in the composition. Draw the outlines of the shadows and use even harder pencil to denote the highlights. Additionally, make sure that you put reflected light onto the neighboring objects to enable cross-talk between the pieces of the composition.
Step 2: Working with color.
As all essential parts are present, you can start imparting the color to the composition. The transparency of the watercolor paint requires the artist to preserve the highlights, rather than painting them, so the first logical step in coloring is to cover the well lit parts of the composition. The general direction you are to take is to move from highlights to the shadows finishing the work with the deepest tone that will make the objects look crisp, radiant, and three-dimensional. The rule universal for any artistic media (and life in general) is to not focus on a particular area of the painting and try covering as many areas at once as possible to ensure the connectivity between the objects and their insertion into the background and surrounding air. As soon as the lit areas are covered, make sure that the layer is completely dry before starting to cover semi-lit regions (note that this relates to the general academic technique only and does not apply to the wet-paper watercolor technique or freestyling). At this stage, make sure to put as much vivid color, diversity, and brightness of the pigment. As you proceed to shape the objects with color and tone moving towards the shadows, increase the density of the paint by reducing the dilution with water. As you approach the shadows that the objects cast remember that the white balance of the highlights and shadows is complementary: if the incident light is warm the lit areas are warm, but the shadows are cold, so make sure to put some blue hues to the shadow color; and reverse, cold illumination produces cold highlights and warm shadows, so red/ochre/umber mixtures are well-suited for the shadows. Not taking this rule of thumb into account will flatten the composition and make it somewhat plain looking. When the general tone work is done, you might want to work on the perspective. We all know that the sky is blue because of the atmosphere gas and moisture particles scattering light. The same applies for the distant objects on the composition. While you want to preserve bright contrast in the closely-located objects, more distant pieces of the composition are separated from the viewer by the larger air mass. Even though in real life the effect of the air density might be hard to catch (it is also not trivial for the eye to be able to focus on multiple planes of view simultaneously), for the painting it is easy to depict. The distant (it applies even if you are painting a single mug on a table and want to show the rear part of the top rim and front side) parts of the composition will look more diffuse and will have more blue-ish tone to them, as if we are to imitate the atmosphere, but on a smaller scale. It is done by using some well-dilute cold blue dye and large soft brush. It will only take several confident and relaxed strokes on the background. Another trick you can apply to diffuse the distant part of the composition is to "wash" it, especially if you happened to be very elaborate on details at the back of the composition. Using the same large brush, go through the sharp boundaries between the color strokes and overlaps, borders of the objects and slightly smear them. It will further enhance the depth of the composition. For the front side, try to enhance the deepest shadows and contrasts. Even if locally you are very happy with "that perfect stroke", it might not work well for the overall perception of the composition as a whole, so critically evaluate the degree of contrast and work the shadows through.
1. During the whole process of working on a composition including the preparational drawing, make sure to check yourself by putting the drawing/painting several steps away from you to see the overall quality at each stage. It is especially critical before you start painting, as it might be not very evident at a closer look that all objects are falling or asymmetrical where symmetry is required, or that the perspective transformations of the shapes are not approached correctly. Additional things that can be done are to flip the drawing/painting upside down or to look at it in the mirror. Human's brain has some inclination to asymmetry that is very hard to trace, especially being unaware of it, so we need to trick the eye and brain sometimes to break their habitual perception scheme.
2. Make sure to use clean eraser, well-sharpened pencils, well-tamed brushes. Wash your hands before painting and try to minimize touching the paper to avoid transferring the skin oil. Change water frequently and make sure the pallet is clean to prevent pollution of the clear pigments.
3. While paper pallets can be nice for testing the colors, they are hardly suitable for sustaining necessary amounts of the paint. Thus, it is highly recommended to obtain plastic pallet with multiple deep wells. These will allow to mix good amounts of the desired colors and consistency of the tones which is especially critical for large-scale art pieces.
4. Use long brush throughout the work, except for the fine detail mastering. This will make your strokes free-flowing and confident.