Hello!
I have no experience in transfering .de domains. But if it's similar to other domains you could do this:
1 - Go to the .de domain name you want ot transfer dashboard, in your Godaddy account.
2 - Check to see if your domain is no longer under the 60 days lock period.
3 - Under settings, ask for your authorization code.
4 - Under settings, unlock you domain.
5 - Send the authorization code to the person you want to transfer the domain to. That person will initiate, with their registrar, the transfer.
6 - Follow the instructions in the following emails you will receive from Godaddy.
Best regards!