The basic concept for my powerplants works like this (actually 3 z-levels, but flattened for this diagram):
+++++++
+ +_+
+W+W+W+
+W+W+W+
+W+W+W+
+ + +
+X+++P+
X P
_ _
+ wall W waterwheel X connected pump P unconnected starter pump _ aquifer channel
The actual plants have more coils, wheels, and manual starter pumps. The spaces past the wheels probably aren't necessary, so it could probably be more compact. It would also be more space efficient if each coil had two or three wheels in a straight line.
I use a channel into an aquifer layer both to obtain water and to dispose of it at the end -- if you don't have one, you could probably make a circular water system. It also helps to have mechanisms to disengage the plant from workloads while you're starting it. If the plant is fairly large, you can also disengage sections of it and use a cascading startup sequence (wait until the first section is generating power, then start the second, then the third...). Otherwise it won't start until the water has made it all the way through the system (it actually consumes more power than it generates until most of the wheels have water under them).
I'm planning on putting a burning lignite bin in a niche in the inflow to make it into a less exploitish steam plant -- vented with a smokestack for authenticity.