25 years ago, I worked for a small ISP (back when there was such a thing.) When I started there, we had one upstream ISP. I was charged with getting us multihomed. I found some tutorials written by Avi Freedman (1). I don't know what I would have done without him. He made an intimidating topic approachable. Thanks to him, I got us a /20 from ARIN and advertised our routes to two different peers. It was fascinating to learn how it all worked. And the more I learned about it, the more amazed I was that it worked at all.
(1) http://avi.freedman.net/
Avi