On this blog article  Terence Eden explains the basics of Activitypub with an example of one page of code.. Start with a domain or a subdomain,  just configure few lines upload the code and test it. It should work.

I try it. I didn’t work the first time. I didn’t have time to know why. I found the code that leads a blank page. I comment it.

That would give me a start to implement ActivityPub in retilo. That would be really nice to receive comments from other AP account. Of course that leads to lot of work but interesting work. Starting by adding comment or RSS streams to nomadic identity.