Error when saving down / Proxy Graphics help

When trying to save from AutoCad 2013 down to 2007, the error says 'This drawing contains custom objects that are not supported in previous versions. These objects cannot be saved to a previous version......". I understand that I need to save this drawing with Proxy Graphics, but not sure how to do so. The help menu was not much help at all, so could somebody please explain? Thanks.

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