Technique

Have a container of clean water, paper, brushes, and pigments ready. You should have a cotton rag or paper towel handy as well, plus a scrap piece of watercolor paper on which to test the pigments you're about to use before you apply them to your painting.
First practice making washes. Watercolor painting involves using washes of one kind or another more than any other technique. The two most common washes are termed "wet in wet" and "wet on dry". Wet in wet simply means painting on an already wet surface of paper. Wet on dry means painting on paper that is not wet. It's dry! Like applying a loaded brush to dry paper, or calligraphy, or using dry brush techniques. Keep in mind that you can adjust the amount of dampness or wetness of the paper. You can even add water after painting with pigments. Bear in mind also that a wash can be any size, from covering the entire paper to painting a small detail like a button on a shirt.

Let's begin with a relatively simple excersize. A one color wet in wet uniform flat wash. A flat wash is a wash that doesn't vary in hue or value. It's uniform over the entire range of the wash. Theoretically the ratio of pigment to water remains consistent while applying the wash.

Grab, cut, or rip an 8 x 6 inch piece of 140 lb. or 150 lb. cold pressed paper. Lightly, draw a pencil border on the paper about an inch from each edge. Use a straight edge or you can free hand it. An easy and fast method to freehand a pencil border is to use your index finger as a guide against the edges of the paper. Freehanding a Pencil Border Demo. Hold the pencil between your index finger and thumb, thusly.

Prepare enough pigment for the wash, let's say cobalt blue. I like using white porcelin trays to mix the pigment and water together. I begin with a few drops of water from an eye dropper in the deep end of one of the tray reservoirs. A few drops of water on the cobalt blue cake will dissolve enough pigment to pick up with a brush. Charge your brush with pigment from the cake and deposit it in the shallow end of the same tray reservoir. Mix the pigment and water together until you have enough to make a light blue wash. Remember that pigments look lighter in value when dry. With practice you'll quickly learn how much pigment and water to use. Squeeze just a little dab into the shallow end of the reservoir if you are using tube pigments, and mix it into the water until disolved.

Wet the paper with a 1 inch flat brush, a cats tongue brush, or a 1 or 2 inch wide Japanese brush. Charge your brush in the jar of clean water. A correctly charged brush won't drip on the way from the water jar to the paper. You may need to lightly swipe the flat side of the brush against the inside of the jar lip to remove excess water. Begin the first brush stroke at a top corner of the paper at the border moving the brush from either right to left or left to right, stopping at the opposite border. Try to keep the brush inside the borders so the water doesn't over lap them. Pigment will always flow where the paper is wet or damp. You'll want to keep the borders as dry and clean as possible. Lift the brush and begin another stroke moving in the opposite direction of the first stroke and slightly overlapping it. At the end of each stroke you may need to recharge your brush with water. If your brush has enough water left after the first stroke you may be able to hold off recharging until after the second stroke. You'll have to judge. Continue brushing with water from side to side until the entire surface of the paper inside the borders is wet. Properly done, the paper should be uniformly wet and without puddles.

Wow! There's your first wash. Now you want to add pigment to it.

Proceed as before, only this time dry your brush on a rag and charge it with the blue pigment you mixed. Brush side to side, border to border beginning at the top of the paper and continue until you reach the bottom border. You'll need to recharge your brush with pigment as you go. Use just enough pigment to apply a smooth even wash. Again, there should be no puddles when you are done. If the wash is lighter at the bottom than it is at the top you can turn your paper upside down and do the whole thing over again just like before. This will even out the wash. With enough practice you won't need to do this. You may find it easier to work on a tilted surface so your washes flow downhill. Be careful of puddles. If you do have puddles, especially at the bottom of your paper, you can mop them up with your brush or a slightly damp cloth.

A gradated one color wash is done wet on wet just like this with one difference. Instead of recharging your brush with pigment as you progress, recharge it with water. This will dilute the pigment as you work thus making the value of the wash lighter as you progress toward the bottom of the paper.

Wet on dry technique
Another technique for making washes is called wet in dry. Simply put, this means brushing pigment directly onto the paper without first wetting it. Washes using this technique will tend to be stronger in hue and value. Brush strokes will tend to be more visable as well, there being no water layer to blend the pigment as it's laid down. The grain and sedimentation of the pigment may be more visable too. Generally, dry on wet is used for washes covering small areas while wet in wet is used for larger open areas such as skies.