To add a new collection at the top level, select My Library, then select the New Collection button. To add a new sub-collection, select the parent collection, then select the New Collection button:
To add a reference to a Collection simply drag and drop it to the appropriate folder icon in the left-hand menu bar. Note that references do not automatically get removed from one Collection when added to another, and that deleting a reference from a collection does not delete it from your library.
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