This is a commonly asked question. It can't be done automatically with Mp3tag, since Mp3tag works with only one file at a time. This makes copying values between files impossible.
Probably the easiest way in Mp3tag to give multiple files a single field value is to:
- Highlight the files.
- Use the 'Tag Panel' (the form-like data-entry panel on the left-side) and enter your value.
- Enter the value you desire for all files.
- Hit 'Save'.
If you happen to be working with either the DATE field or the GENRE field, you have the option of selecting an existing value from the drop-down box. Once you hit 'Save' you'll have effectively copied the selected value to all files that are highlighted.
I find the easiest way to work on all files in a folder (in my library this is typically a single album) is from Windows Explorer, right-click the folder, then select 'Mp3tag' from the context menu. As soon as Mp3tag loads the files, hit Ctrl-A. Use the Tag Panel to edit any fields common to all files in the album. Hit Save. Use the file view (on the right) to edit any individual fields, such as TITLE. When done with the album, close Mp3tag and then go on to the next from Windows Explorer. I never use the folder navigation from within Mp3tag itself.