You're right that "here are how" is correct, for the reason you give. [This article][1] goes into more detail.

In written language, you should normally write it this way. However, I don't think most people would say it that way. I would definitely say "here's how" because it's shorter, and I don't think I'm the only one. So, if you're trying to reproduce dialog, you can use "here's"; otherwise, use "here are".

[1]: https://www.wordnerd.co.uk/words-language/is-it-heres-or-here-are-and-why-you-should-care/