Good question. I, for one, still struggle with making a flat wash. It is a matter of practice and getting to know how the paint moves. This is what I know about how it is done, doing it right is another thing
. Anyhow, dont have you paper flat but a bit slanted so gravity helps you. Make enough of your color to cover the area you are painting This is important. Use a big brush to cover as much as you can when making a stroke. Starting at the top paint left to right,then right to left, etc .You have to paint rather quickly as drying paint will cause lines. It gets a bit tricky when there is an object and you have to paint around it. Or you can use masking fluid and continue the stroke across. Look on You tube for flat washes and you will find some videos I am sure.
