This is the total list, the top stuff you already know:
Remove the dirt.
Lay down underliner/geotextile as farmtek calls is "non woven pond underliner".
3 inches process (gravel with sand).
3 inches gravel.
THEN, in Ms. Java's pen - mats with holes.
She is the destroyer of all footing.
She got mats, then more gravel.
Actaully, all the pens got more gravel.
We got rain today, great excuse to use the pens while the grass paddocks dry out. Since it's 80* and sunny, that won't take long.
The layout again is like an L. each is 12x24. If we want to open a pen larger without all 3 out, we can just open an gate and give that horse more room: