Via contact form you can upload your song to my webserver. In addition you can also send a reference song if you already have precise ideas on how the mastering should sound in the end. Important: Depending on the file size and internet speed the upload may take a while!
If the upload does not work, you can send it by email (firstname.lastname@example.org) wetransfer.com or any other file transfer platform. The postal service is also possible, the address can be found in the legal notice.
More information about the process can be found here.