Expanding the circle

  We start with the "Older Heaven" bagua. Always read the baguas from the middle of the circle.

Rotate it to have the "Mountain" on the top of the circle.

By mirroring it outside, we get the eight "great" hexagrams. If we merely duplicated the circle, we would only have "Mountain over Mountain", "Wind over Wind" etc.

Now make a mirror image inside:

Rotate the inner circle 90 degrees clockwise:

Finally, write the sequences "Heaven-Earth-Water-Fire" and "Heaven-Earth-Fire-Water" alternating on the four sides.

That's how the sigil was built.