GDPR Double Opt-In Documentation

Since it is necessary to keep a log of the double opt in subscription service I would like to ask which options have to be set to make this log exist and to have it easely downloadable. Do I have to reach for the log-files in the content folder via FTP or is it possible to maybe "oneklick" this in the wordpress backend? I hope I was able to make my problem clear. I hope I can find a log file in which the information is time stamped when a user signed on and if he or she accepted the double opt in process. German Law - sorry.