Lot of articles re-hashing NIST 800-63 since it came out last month. Here's the original source: https://www.nist.gov/itl/tig/special-publication-800-63-3
In fairness this is more then rehashing NIST 800-63. This is coming from a Google engineer who worked on their authentication systems. A lot of good advice here, starting with not building your own.
Oh, so basically scripture then.