This is a sample config to bond 2 T1s together to form a single 3Mbps circuit:

interface Multilink1
 ip address 1.2.3.4 255.255.255.252
 load-interval 30
 shutdown
 no cdp enable
 ppp multilink
 ppp multilink fragment disable
 ppp multilink group 1
!
interface Serial1/0
  bandwidth 1544
  no ip address
  encapsulation ppp
  load-interval 30
  no peer neighbor-route
  no fair-queue
  ppp multilink
  ppp multilink group 1
!
interface Serial1/1
  bandwidth 1544
  no ip address
  encapsulation ppp
  load-interval 30
  no peer neighbor-route
  no fair-queue
  ppp multilink
  ppp multilink group 1

It is the multilink1 interface that is actually the single combined interface that really has the “3Mbps” of bandwidth, and that is where you would apply all of your “normal interface configuration” for this 3Mb connection that connects to the Internet. You can see how the multilink 1 interface has an interface number that matches the ppp multilink group. On that multilink 1 interface, you have the IP address for this link, an ACL on the interface, firewall, NAT, or other Internet public router interface commands.

To verify that your multilink interfaces are up and running, you can use the show ppp multilink command, like this:

Router# show ppp multilink