The unprocessed documents collection will vary depending on the context in which you run your agent. However, the basic process is fairly simple. If you are only going to process one field, check out the stampall method of the NotesDocument class, it's much faster than changing the field and calling save yourself. The following sample LotusScript code should get you going, it could be used to process the selected documents in a view.
Dim nsCurrent As New NotesSession
Dim ndbCurrent As NotesDatabase
Dim ndcCurrent As NotesDocumentCollection
Dim ndocCurrent As NotesDocument
While Not ndocCurrent Is Nothing
'Process document here