As someone above stated, you can use static routes if you so choose, but other routing protocols can be used as well. For an easy setup, we can use a classful routing example with a standard class C mask (255.255.255.0). Typically you wouldn't want to do this due to lost address space, but to get the concept across we will.
For the router labeled 0, we could use an ip address of 192.168.1.1 /24 (255.255.255.0 sub mask). Remember that serial links between routers are on their own broadcast domain, so you have to choose an address within the same subnet of the first link. Since we have a HUGE (and unnecessary) amount of address space we can use 192.168.1.2 - 192.168.1.254 (Max of 254 addresses per subnet with a /24 mask). Use any address within the range, but I will use 192.168.1.2 for this example.
For the sake of the lesson, lets just continue using /24 masks. The link between router 1 and 2 could use an address range from 192.168.2.1 - 254 for example. Okay so now we have to configure routing protocols on EACH of the router. Think of it as telling the router where to go next to get its information. You could use any type of protocol, but honestly, it just depends on your preference.
You can do static routes. On router 0 we can set a static route to router 2:
ip route 192.168.2.0 255.255.255.0 192.168.1.2
This means the router will forward traffic to the 192.168.2.0 /24 network and will exit the router from the interface with the ip address of 192.168.1.2 .Why did we not have to set a ip route for the links between routers 0 and 1/ 2 and 1? Well, because they are directly connected, they can already reach one another.
If you want to get REALLY easy, you can always do default routing which is just using the command (using router 0 as an example again):
ip route 0.0.0.0 0.0.0.0 192.168.1.2
Which is basically telling the router you don't CARE what network is out there, you know it is out there somewhere and the only way to find it is through 192.168.1.2 . You can use other protocols (OSPF, EIGRP, etc) but that is up to your network's needs and preferences.
Hope this helped! (sorry if long winded)
