Populate custom fields for a single file via the file options ellipsis:
Click the file options ellipsis.
Select Edit Custom Fields.
Populate each field according to the field type.
Click Apply.
Populate custom fields for a single or multiple files via the bulk actions button:
Select one or more files, either individually or by checking Select All.
Click the caret on the bulk actions button and select Edit Custom Fields.
Populate each field individually, copy and paste values between fields, or select and drag the bottom right corner of a populated field to apply the same selection to multiple files.