This is a not so detailed solution to how to go about this task. The following is an edited version of the description by an networking Guru on how to go about this task. Who goes by the username steve.freke on techrepublic. I have added some stuff of mine onto what he had shared .
Use the OSI model. Define what it was layer 1 – that is, all
the physical connectivity between the active devices on the network.
This includes cable runs between server rooms and wiring closets. This
step will take the longest and is the most often ignored because it is
the most tedious.
Move up to layer 2 – document collision
domains, STP instances, etc. Move to layer 3 – document broadcast
domains, subnets and routing instances, including...