The FEP-ef61: Portable Objects describes how to use DIDs with ActivityPub. Here's a slightly less technical introduction: https://codeberg.org/ap-next/ap-next/src/branch/main/nomadpub.md

It's not easy, though. Adding this feature to an existing project will require a lot of work, especially if you don't want to share signing keys with servers. This was discussed in #3100, Lemmy devs are not opposed to FEP-ef61, but they don't plan to work on it.

Also, I don't recommend copying solutions from ATProto, their did:plc and did:web are not really "decentralized".