Begin by mixing a color of your choice in a small bowl, so that you have enough to cover the page. Tilt your board (on which your paper is fixed) about 10 degrees. Just a slight slant so that the color will pool at the bottom of each stroke and can be picked up by the brush on the next pass.
Dip the brush in the color mixture, and begin at the side of the paper and drag the flat edge of the brush across the page from edge to edge. Standing up best does this so that your arm moves freely.
Dip the brush into the mixture again, and allow the top edge of the brush touch the bottom of the previous stroke, so to pick up any pigment that has pooled at the bottom. Drag the brush across the paper again.
Continue this routine until you have reached the bottom of the paper.
Tips: Be sure that the mixture you have put in the bowl is completely free of lumps of pigment. Otherwise, you will have dark streaks in the wash, as they release from the brush bristles.