The Network Documentation Process always starts with an inventory of your assets.
That means you have to identify every piece of hardware that builds and connects to your network. To do that you want to use a tool that will ping and list all the IP addresses in your network and possibly give more information like the name and operating system of that device.
You will find out that there are a lot of tools to do this, but my favorite one to start with is Look At Lan (there is also Fing from the same developer, but I prefer LookAtLan).
It is a simple tool but it will provide you with a great overview of your network. Because it only sees the devices that are really active on you network at that moment you will need to run it again once more users are online and their computers are active, but to find out about your infrastructure it is a good start.
Most of the devices that connect to your network and make your network will be active at each scan:
They are the ones with static addresses or for printers they could be in a separate DHCP TCP IP Pool.
Make a note of that IP Addresses and start from there to build your network documentation.
I start with an Excel Spreadsheet and Document every IP Adress, Device Host-Name, Location “Serverroom A, PatchCabinet Rackmounted in Position 3” and a Description like “Cisco Catalyst Switch 3524 XL”.
Of course you have the label the devices with there Host-name for faster troubleshooting.