I am going to assume that you are running BGP with your 2 ISPs.
I am also going to assume that you want a hot/standby and not a load sharing scenario.
I'll call the router connected to the primary ISP R1 and the secondary to the fallback ISP connection R2.
The primary ISP is A and the secondary is B.
R1 and R2 can accept defaults from their respective ISPs.
You can have R1 and R2 in an HSRP configuration and have the backend routers point to their HSRP VIP for a route to the Internet.
R1 will be the primary and can use interface tracking within HSRP to decrement its priority in the event that it suffers a WAN link failure to ISP A. In that case, R2 will become the primary router for Internet access.
This will establish a deterministic scenario for outbound traffic.
To ensure that inbound traffic from the Internet uses your primary ISP, ISP A, and primary router, R1, to get back to your enterprise network, you can have R2 advertise an extended AS path to ISP B, thereby making it very unattractive to ISP B and its peers, forcing them instead to go through ISP A.
By the way, it's not easy. There's a lot of things to take into consideration, so don't beat yourself up. :-)
We are pleased to announce availability of Beta software for 16.6.3.
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