dig a hole that isn't in clay. How deep and wide the hole is depends on how much water you are working with. For small field or foundation drainage I think a hole about a meter deep and meter wide will work. Then you fill with loose rubble. Large rubble on the bottom and smaller rubble on the top. You can cover it to keep the wind from filling in the spaces between the rocks with dirt. If you are draining a larger area it's better to dig a wider hole than a deeper one.
To build a simple soakaway, you need to dig out a dimensional leveled patch of ground. Fill it with attractive stones and lay a slab on top. When the water overflows, it will immerse through the attractive stone and into the ground.