The limb plexuses are complex aren't they. Personally I found it really helpful to think of it as wiring and to map the wiring from the spinal cord, the primary divisions, the cauda equina the plexus and keep going till you find where each nerve ends up supplying. Like a three dimensional circuit board. so you have to map it out, thinking about it's relationships to each anatomical structure it passes by. When you work out the course of each nerve through the various divisions really understand what function it provides. For example how does the divisions relate to the the extensor muscles, the flexor muscles or which ones are supply the joints etc.

I don't know if this is very helpful but that is how I learnt it.