So here is the process that I use to work my mandalas with symmetry. Mandalas don't need any symmetry as they can be as freewheeling as you want, they are an expression of you!! I love to put symmetry in mine as I have always been fascinated with math, geometry and now sacred geometry and fractals.
So I use a circular grid as I stated in my last post. To get an idea of how I use the grid, I worked the beginning of this mandala with the grid still on the paper in pencil. You don't need this complicated of a grid, but it gives you an idea of how I form my framework