To braid the dough, you take the whole dough from the bowl and put it on a well floured surface. Gently flatten the dough and make a rectangle. Additionaly, try not to punch or knead the dough, since this will damage the structure and will result in a flat(ter) brioche. The larger you make the rectangle, the longer and lower the braid.
Subsequently, with a pizza wheel or dough scraper, divide it into three equal parts. Next put the top of the three dough strings together, making the stick. After you finished the braid you can put these under the braid, that will look prettier.
Continue to braid the dough untill all dough is braided. Stick the last three strings together like you did at the beginning and tuck them under.