Notice how slowly I'm working. There is no need to rush if you have big bead of paint to work with. If you take your time you will not miss bits and the paper will have the chance to soak up the paint. By working in this manner there will be no need for you to go back and correct areas that have been missed. Doing so invariably leads to back runs that ruin a perfect flat wash.
Notice also that I paint right off the page and onto the brown paper. This helps the paint move away from the wash preventing back runs.
I also demonstrate how to make a "thirsty" brush. I do this by removing the paint in it by wiping it on an absorbent paper towel. I then use this thirsty brush to remove the bead of paint at the bottom of the wash. A thirsty brush works by osmosis - there is less paint in the brush than on the paper therefore the brush soaks it up. This is an important point to remember when learning to paint. It is a technique that is used a lot. If there were more paint in the brush and less on the paper the paint would come out of the brush and create a back run.