It's really just a process of listing what you have, then listing what you need. Then using the first list (your assets) to build out a network that does what you need. You may have to buy things you don't have, to create the network which meets your needs. Like so:
- First you make a list of the sites.
- For each site, you make a list of what's there.
- If possible, you list out what will be at each site in the future. This will help you plan a network that meets the future needs as well as current ones.
- If you already have connections between sites, list these and their bandwidth.
- HQ: 50 desktop computers, 8 servers, 3 networked printers. 1 VPN concentrator, 10 mb/s internet connection.
- Site1: 15 desktop computers, 1 networked printer. 1.5mb/s ADSL internet; VPN to HQ.
- Site2: 23 desktop computers, 2 networked printers, 5 POS stations. 6mb/s business cable to internet; VPN to HQ.
- Site3: 17 desktop computers, 2 POS stations 1 networked printer. 1mb/s business cable to internet; VPN to HQ.
- Site4: 10 desktop computers, 1 networked printers. 768kb/s business cable to internet; VPN to HQ.
- Site5: 37 desktop computers, 2 networked printers, 2 servers. 6mb/s business cable to internet; VPN to HQ.
- Laptop Users, mobile: 12 mobile users, each with home broadband or working from Starbucks. VPN to HQ.
You probably want to draw this as a map (lots of examples here). Print a couple of copies and then play on it with a pencil. Think about the connections which need to be made from site to site, and how much bandwidth they will use. Example: each desktop user might have a 50mb/s connection to the mailserver every 5 minutes. The servers at Site5 keep a continuous 1mb/s stream of replication traffic going with the servers at HQ. And so on. You'll get these estimates wrong, but they are a starting point that helps you see if you have enough bandwidth between sites. Later you'll put a little monitoring in place to get real numbers. But now I'm getting ahead of myself!
Next up you need to think about your IP addressing and subnetting plan. Will you be using public or private IP addresses? What the maximum number of IP's each site will need? Get out your subnet calculator and figure up how large each site's subnet will be. Decide which specific IP range will be used at each site. From here you'll decide what each site's internal gateway address should be (the external IP's in this example will be decided by the broadband provider, of course).
Now you decide whether you'll use DHCP to handle all those addresses (you very likely will). It will need to be configured - and you'll have to decide where to run it. At HQ, with ip-helper addresses at each site's router? Locally at each site from the routers there? Draw this into your diagram.
It's time to figure out your name services. DNS most likely - where will you put it? You could put all DNS servers at HQ or you could distribute them to each site. Once you've figured that out, add it to your DHCP config.
And so on, and on, and on. Basically you start with what you know, put it on the map, then start trial-fitting what you want on the map, until you have something that works. You commit your pencil annotations to ink. Then you build it.
Then you run it for awhile, and make changes as needed. Which you (or someone anyway) will keep doing until the company dies.