A mandala uses rotational symmetry. Whatever you draw in one slice gets repeated, rotated, around a centre. With 6 slices it looks like a snowflake; with 12 slices, a stained-glass window.
How to make one
Pick the number of slices (4, 6, 8 or 12 work well). Lightly draw the lines of the slices on paper. Decorate ONE slice — then carefully copy that decoration into each of the others, rotated to match.