I am not sure why you would want to do that as its better to have both routers active.
The easiest option with BGP is to setup your two routers running HSRP and each with the same upstream BGP session with the ISP. The ISP has one session back to your HSRP address and the primary HSRP router will only establish an active session until it goes down and the standby kicks in.
Many ISP customers have done this before but I dont like it as BGP take time to establish and accept prefixs so it suffers an outage for several minutes.
This also assumes an ethernet connection.
My advice would be have both sessions up and iBGP between them and local preference on the one you want as primary.
