In the notes, first cite the specific archival record, followed by the date (day, month, year), identifier (box/folder/item number), name of collection, name and location of repository.
In the bibliography, first cite the author, followed by the specific archival record, followed by the date (year), identifier (box/folder/item number). Next list the name of collection, followed by the name and location of the repository. Also: cite individual items when you've referenced only a single item from a collection.
If you reference multiple items from a collection, cite the collection as a whole: Collection name, reference code. Repository, location.
In the case of an item that is both a paper document in the collection and a digital object in Archives Online or Image Collections Online, the best way of citing the document as a digital object in APA is to add the Reference URL to the end of the APA citation preceded by "Retrieved from".