First I work simple shapes onto the grid. Just follow your flow. I go back and work in more detail later. I go back many times, as the mandala usually speaks for itself, and it is like working layers into a piece of art. Everytime you finish one layer, you can see where to start with the next one. So start simple and just sketch in some shapes. I do them in pencil, as I will change a shape into something more detailed or overlap a new shape over the existing one. Sometimes the shapes are simply a frame for a more detailed drawing.