It’s useful (in some cases) to have a functional piece of software before adding in federation. Certainly with Lemmy as a (relatively-)new space for Fediverse software. If you were making a microblogging platform, maybe starting with federation is the better move.

But basically you “just” need to take your existing software and (a) publish activities based on actions happening on your instance to some process that can durably fan out to the other instances and (b) add some new endpoints for processing incoming activities from other instances.

Of course the complexity comes in the actual implemention details it’s not easy by any means for a non-trivial piece of software, but based on the AP and AS standards there are at least well-defined APIs you need to implement.