Hello,
first of all, I assume you already have a leased line or some other connection between your building ?
Basically, and assuming that the link between your routers is a point-to-point leased line, here is what the configuration of your routers would look like:
Router A:
interface FastEthernet0/0
ip address 192.168.1.1 255.255.255.0
!
interface Serial0
ip address 192.168.3.1 255.255.255.252
encapsulation ppp
!
ip route 0.0.0.0 0.0.0.0 serial0
Router B:
interface FastEthernet0/0
ip address 192.168.2.1 255.255.255.0
!
interface Serial0
ip address 192.168.3.2 255.255.255.252
encapsulation ppp
!
ip route 0.0.0.0 0.0.0.0 serial0
This is very basic, to start out with, let me know if you need any more specifics.
Regards,
Georg