You start the transfer via a banner in the goFLUX app.
The Karos app is only needed later in the process.
Make sure your goFLUX app is up to date.
Open goFLUX and tap the banner "Transfer my account".
During the process, you can either sign in to Karos using you existing login details or create new ones.
In both cases, your account and all your data will be fully transferred.
⏸️ Here's a video tutorial for the transfer process: