Minecraft wiki entry for Endless_circling_pool, associated text: You can replace the stone bricks shown in the grids above with any solid block, such as cobblestone or wood planks.
The slab can be of any type. It can be any block one automatically steps up onto, that water cannot wash away, meaning that stairs are another option. Place the slab or stair under the water source that is 2 blocks high.
The water source block (marked with an S) flows down, and around to under the sign.
After the water is flowing correctly, the sign can usually be removed once everything is in place.
1.13 has changed the water flow mechanics, such that the end of the water has less current. This can cause you to get stuck on the central block. Replace it with a sign instead to solve this issue.