There are a few factors that will help you get the smoothest flat wash.
even moisture on the paper; get it nice and evenly shiny
the right paint. Mix up a nice puddle of "inky" paint, and use a non-granulating pigment.
patience and attentiveness. Tilt the paper to allow the paint to flow, and swipe away excess moisture from the edges. Don't walk away until the paper's sheen is gone, and leave the board tilted slightly through the drying process.