Ey gude wie, there's this guide you could follow, it's from Molly herself:
https://github.com/mollyim/mollyim-android/wiki/Migrating-From-Signal
(I'm new to interacting with posts on mastodon and I didn't know if I should leave the other accounts from the thread or not. Is there an etiquette? 🙈)
@pepello @marcel Welcome! I wouldn't say there's an etiquette, but since the default is that everyone in the chain was included, I usually keep everyone unless I'm effectively starting a new and unrelated conversation.
If a thread gets really popular, the OP can (and often does) end up muting their notifications if they decide to bow out, and Mastodon clients tend to emphasize making that easy.
In short, do what makes sense to you!