Draw a network diagram using the guidelines attached in these PowerPoint slides. After you get your diagram built, then at each node on the network write about how an attacker can attack that piece of equipment and what damage it could cause to the network (how serious it is or could be).
You'll have a paragraph or two for each of these pieces of equipment:
Firewall
VPN
Proxy
Router/Switch
Depending on your network, there is a good chance that not all of these types of equipment (or software) might be present in your network.
If so, then just write from experience and what you learn from the book about that type of equipment.