When you draw a mandala, you have to be in a quiet place, which you can draw without disturbs, sometimes you will feel to listen to some music or some mantras
Focus in a problem that you want to solve and visualize it. Once done, allow yourself to choose the colours without looking at them by closing your eyes, for example.
Don´t move the paper while you are painting, paint the mandala without moving from the place, I mean that you can´t leave something for tomorrow.
Paint from the inside to the outside and put a mark in the paper to know what is up and down.