1. Go to https://github.com/benediktg/gnu-social-export (thx @benediktg), clone repo, follow instructions to run commands. 2. You get a following_accounts.csv file. Go to your new (Mastodon) instance (<instance-domain>/settings/import) and import the follower list. Verify that the followers were imported. 3. Change your avatar/bio/username at GNUSocial so one sees this account is inactive. Point to your new account there. AFAIk mentioning yourself in a qveet (@username@newinstance) works in a good way, too.
@benediktg 4. Go to https://gnusocial.de/<username>/subscriptions and unfollow accounts. 5. Frequently re-post your migration message/post at GNUSocial.de, so the ones following you will see it in their timeline. They can (and should) unfollow you once they follow the new account. 6. Revoke your authentications done in https://gnusocial.de/settings/oauthconnections. This just feels good… 7. At the last day or so (May 31) you may delete your old account at https://gnusocial.de/settings/profile (on the right).