I wrote a blog post with step by step instructions:
I know it. You're using Protonmail in the post.
But overall you're a Fastmail user. So why recommending Protonmail and using Fastmail?
Personally, I chose Fastmail over Protonmail for features and pricing. I didn't need the PM->PM encryption that Protonmail offers, at least not for my primary email. Fastmail was more feature-rich everywhere else.