|You enable it and configure it.|
How you configure it depends on what you have running....a domain or a workgroup.
If you have a workgroup, I recommend using the DHCP service on your SOHO router.
Configuration of it is simple enough, define your scope, set the gateway and DNS IP's and that should be all you need.
Lets say you're using 192.168.0.0/24as your subnet. and you're going to reserve the first 100 IP's for static applications such as servers and printers etc.
Assuming your server's IP is 192.168.0.10 and this is an AD domain with the server as your DC. Your SOHO router's IP is 192.168.0.1
Scope: 192.168.0.100 to 192.168.0.200
DNS: 192.168.0.10 (forwarded to your ISP's DNS)
Default Gateway: 192.168.0.1
Subnet Mask: 255.255.255.0
You'll notice your gateway IP is your router as it is the :"gateway" to the external network. The SOHO router should be properly configured and working prior to setting up your domain.