Hi,
You first need to decide what section you want to split. (Clean analog, digital, power, maybe smps. ..)
Then think about placement. Maybe you have some placement restrictions. What size do the areas need?
Then you should think about (pulsed) high currents. How to handle it not to interfere other signals
Then about the devices and the signal paths that "connect" different sections. Like ADC, it needs signals to the digital section, needs clean analog gnd and supplies, but maybe needs a clean path to the power section...
I often use only one gnd plane and place splits. Usually a 270° or more split around a switch mode supply circuit.
Another member had a good picture. Imagine the Gnd plane as the surface of a lake. Throw a stone into the lake and see how the waves behave. Build imaginary walls (splits) to keep the waves in a restricted area.
Klaus