A flat wash is in watercolor a completely flat painted surface without visible brushstrokes. The traditional way to achieve this is as follows:
Mix so much paint so that you are convinced that it is enough for the entire surface to be covered. It is not possible to mix more when needed, partly you do not have time and partly it is not possible to get the same color again.
I usually say that you should aim for twice as much color as you think it takes, then it will probably suffice.
Tilt the paper. Some watercolor painters tilt much others less. How much you tilt is a matter of taste. Too little and you risk uneven color, too much and the color can let go and run all over the paper.
Fill your large brush with paint, it should be properly filled. Start painting at the top of the surface to be covered, as the paper is tilted, the paint will run down and accumulate in a long bead.
Gradually move the long bead downwards over the paper with long horizontal brush strokes. Dip the brush in paint continuously, it must NEVER get dry, there should always be a bead at the bottom.
Excess paint at the bottom of the surface is soaked up with a damp brush, never with a sponge or paper.
You can not change and fix on already painted surfaces, if you see a mistake somewhere when you are done you have to leave it, it will be an ugly stain if you try to fix.
It is gravity and the surface tension of the water that do the job, you just need to provide the paint.
This exercise must be completed by all participants in my courses. Almost everyone fails at it, always for the same reason: Too little paint in the brush! It’s obviously very difficult to use a lot of paint, I do not know why, but it is. Once you have filled the brush, it should be directly on the paper, it should not be drained on the edge of the paint container. You can not have too much paint in the brush.
Once you are comfortable with this classic way of doing a wash, it’s time to discover all the other ways. You do not have to tilt the paper, it can be painted with brush strokes other than horizontal. You can dilute the paint gradually, and create a graded surface. You can also change the hue gradually from blue to red, for example.