Well there are optimal ways of doing(layers, good mixes, etc). But bioactivity on itself almost always happens on it's own with soil. So you just have a layer of soil and over time aerobic(beneficial) bacteria will cover it and most likely springtails and other micro fauna will appear. Obviously, many of us speed this up by making nice mixes of soil/leaf litter/etc, adding springtails/isopods/etc, possibly adding some outside soil(from a clean source). Bioactivity is a simple and natural process with organic substrate. If you happen to be familiar with aquariums(specifically marine), it is a very similar concept regarding the cycling of tanks and seeding with microorganisms.