The format is a simple text format of the form.

<metadata_type1>:Attr 1
#Some comment
<metadata_type2>:Attr 1:Attr 2:Attr 3
...

To flag a metadata property as inherited do:

<metadata_type1>:inherited:Attr 1
...

The metadata type IDs can be viewed at /metadata/types.html

content.keyword:Keyword 1
content.keyword:inherited:Keyword 2
#Some comment
content.timezone:ET
archive_note:inherited:Some other note type:Some note
#etc...

Go to the entry you want to upload metadata to and select "Add Properties...". You should see a "Upload" link which takes you to a file upload form. Your file should end with ".txt".

The upload process will skip over duplicate metadata so you can upload the same form repeatedly if there are any errors or missed metadata in the upload.