Pretty much like this: Re: Bonding multiple T1 in Adtran 3305 with additional NIM
The document isn't 100% clear, you'll want a cross-connect from each of the T1 interfaces to the ppp interface (document just lists one).
This assumes that AT&T is using PPP as the protocol. If frame-relay, it's similar but with a freme-relay interface and a few added commands for LMI-type and DLCI.