Export all covers to a file with a decent filename (e.g. %album%.jpg).
Try to find out if you have any fields with linebreaks in the data (COMMENT and UNSYNCEDLYRICS are likely candidates.)
If you have fields with linebreaks try to replace these with a printable character like #.
Then determine, which fields you have in the files (so that you do not loose any information).
Then select an export format that is as close to plain text as possible and that has a printable separator between the fields. "CSV" is almost perfect but does not like the linebreaks.(if you have replaced the linebreaks that is fine - I know that there are suggestions in the forum to do the replacing while exporting but that may be a discipline for advanced users and not the first try).
HTML is not a good as it adds formatting information to the fields as does RTF).
Use the CSV-Template and add fields that are in your files but not included in the template.
It may be a good idea to note the sequence of fields in plain text as the first line in the export template.
Also, include the filename as field as this would then help to assign the correct data to the right file during restore.
To restore the data, first edit the export file and cut the first line - this line would confuse the import.
In MP3tag load the files and select them.
Now use the Converter Text file - Tag.
You are then requested to enter export-filename and the sequence of fields and their separator - for this you may use the cut line from the export file which should still be in the clipboard.
Blanks between separators are treated as data during export.
For import there must be no blank between the field variable names.