I'm trying to set up a redundant connection for our web hosting servers. I have a BGP-capable router connecting two lines from two separate ISP's, one fiber (primary), one DSL (failover). I've already confirmed that I can push routes into the DSL ISP's system, so long as they aren't malicious. My question is, what do I need to do on the fiber side to make those IP's routable through the DSL? Is there something I'm missing?
At a high level you'll need several things to get started.
1.) A BGP autonomous system number. You'll need to register for one with your local registry (ARIN / APNIC / RIPE / etc).
2.) A routable subnet - either a sufficiently large allocation from your primary provider (generally at least a /24) or provider-independent space.
3.) For full redundancy you'll need both providers to accept the route. If it's not PI space (likely) then the provider who owns the supernet will likely need to allow you to leak your more specific prefix past their summaries.
In practice the above generally requires a substantial amount of knowledge around how Internet routing works as well as the particular routing platform you're using (BGP 'capability' may not sufficiently address what you need, for example). Perhaps more to the point it's going to require a degree of service and cooperation from your upstream neighbors. Keep in mind that the ability to advertise a given route doesn't imply that your carrier's peers will accept that route. It's been common practice for a very long time to not accept prefixes longer than, say, a /24 (occasionally even shorter).
All this aside, however, what you've posed isn't a discrete question. You may want to do some research and come back with more specific inquiries.