Harder than implementing RSS. Less hard than writing Linux from scratch. Where it lays in the middle depends a shitload on your requirements. How much you want to federate (a blog post VS conversations for instance), do you try for exact compatibility with existing services VS using a different activity type, is it one way or bidirectional, how does it need to scale, etc. I don’t know of a generic library you can use so you’d need to read and understand the spec.

There is a crate which maintained by @nutomic@lemmy.ml: https://crates.io/crates/activitypub_federation

I believe it should be compatible with other projects 🤔