Make your lemmy accounts through lemmy.org because it doesn't require an email. Use a VPN (lemmy.org is fine with that). Use an anti-fingerprinting browser or TOR and be careful not to add extensions that will make you uniquely identifiable. Maybe check out Mullvad Browser or Librewolf? They both have letter-boxing options that will keep your screen at a standardized resolution so you can't be fingerprinted easily that way.

Honestly, depending on how oppressive and competent the regime you live under is, it could be too much to do this safely. I don't know where you're posting from so I don't want to give you any false sense that you can do the above options and be free and clear.

EDIT: Also don't sign into things on your TOR browser that you sign into on other browsers. Keep it as an entirely separate identity.

And don't have two browsers open on the same site and move your mouse from one to the other. There's a lot of little things that very driven people can do to find ways to identify you...