In the beginning...
Draw a square or rectangle on your paper, or visualize the boundaries of such as you go. (wing it)

Select a darker hue for your wash (it's easier to see) and mix a liberal amount of medium intensity (30-50% value) paint on your palette. I'm using a 1 ½" (381mm) Winsor & Newton Series 965 flat wash brush and Holbein Sap Green watercolor paint for this lesson. The paper is Arches #140 CP.

Charge your brush with paint, and starting in the upper left corner touch your brush to the paper and gently pull a straight line of paint to the upper right corner.

NOTE: If left handed work right to left!
Make your second stroke
Return to your palette and refill your brush.

Start the next stroke at the bottom of the first stroke, being sure to overlap the bead of paint now formed at the bottom of the first stroke.

TIP 1: If the flood of the first stroke doesn't fully flow into the new stroke, increase the angle of your board to aid the flow of the wash.

TIP 2: Increasing the angle of your work also increases the chances of drips running wild down your paper. If they annoy you, work faster or keep a tissue or damp sponge in your free hand to quickly blot them away.
Repeat as necessary...
Refill brush and continue overlapping strokes, riding the flow of the paint and keeping an even tone as you go.

TIP 3: You can use the flat edge of a wash brush to "cut" the starting edge.

TIP 4: If you want to square up the final edge of the stroke—slow down, pull the brush up, and use the sharp flat edge again. Pull it up to your line and "cut" the final edge with a downward pull.

TIP 5: If your stroke breaks up, load your brush and repeat the stroke IMMEDIATELY. See (Tip 7) below!
Almost there, keep going!
Repeat steps making stroke after stroke to the bottom. Try to keep an even tone as you go.

TIP 6: You would not believe how much variety there is in the behavior of different brands and grades of paints and papers. The more expensive well-known brands usually make your work easier by offering consistent high quality.

TIP 7: If your strokes break up and your brush is fully charged, you are either using a rough textured paper or the paper could be heavily sized. If you find heavily sized paper like this, spray the paper, sponge it off with a clean damp sponge and let it dry before use. The surface will now be more receptive to your paint.
I've painted something!
Rinse your brush out in clean water and blot or squeeze out the excess the water.

Carefully pick up the bead of paint that runs across the bottom of the wash using the wick action of your brush. If you draw up too much paint you will lift the color off the paper.

Let the wash dry. If you've ended up with an even-toned square of color, congratulations! If not, try it again. I did. And do.

TIP 8: Try practicing your flat washes with different colors and intensities. Each color has it's own physical properties that affect how they feel and flow in washes.

TIP 9: For a pronounced texture in your wash let it dry at an angle. The pigment will settle out in the texture of the paper.

Статья с сайта Watercolorpainting.com