Constructing sink bowls in minecraft

The player called dataless is well known for his enormous farms. Many of these feature a giant pool of water at the bottom that collects all of the farm's drops and funnels them to the center point.

I can't figure out how these sink bowls are constructed. He has one lets play that shows him building a small one with only one level, and he mentions they're very tricky and it's all too easy to turn the whole thing into source blocks. In any case the procedure he shows isn't well explained and I've never found any good tutorials on this, and especially not the multi level ones (his cactus farm uses pools that fall several levels down, with a total width that must be over 25 blocks.)

Does anyone know how to build these, preferably to an arbitrary size? I know there are probably particular dimensions that need to be followed but I'd like to be able to add any number of rings to make the pool larger.

Dataless's Cactus Farm

Constructing a smaller sinkbowl