From James Olney's "'I was born': Slave Narratives, Their Status as Autobiography and as Literature" and other essays in The Slave's Narrative, ed. Charles T. Davis and Henry Louis Gates, Jr. (New York, 1985).
Note: Olney's essay has a great deal more information than this and should be read in its entirety.
The following appears on pp. 152-153.
The conventions for slave narratives were so early and so firmly established that one can imagine a sort of master outline drawn from the great narratives and guiding the lesser ones. Such an outline would look something like this:
A. An engraved portrait, signed by the narrator.
B. A title page that includes the claim, as an integral part of the title, "Written by Himself" (or some close variant: "Written from a statement of Facts Made by Himself"; or "Written by a Friend, as Related to Him by Brother Jones"; etc.).
C. A handful of testimonials and/or one or more prefaces or introductions written either by a white abolitionist friend of the narrator (William Lloyd Garrison, Wendell Phillips) or by a white amanuensis/editor/author actually responsible for the text (John Greenleaf Whittier, David Wilson, Louis Alexis Chamerovzow), in the course of which preface the reader is told that the narrative is a "plain,unvarnished tale" and that naught "has been set down in malice, nothing exaggerated, nothing drawn from the imagination"--indeed, the tale, it is claimed, understates the horrors of slavery.
D. A poetic epigraph, by preference from William Cowper.
E. The actual narrative:
F. An appendix or appendices composed of documentary material--bills of sale, details of purchase from slavery, newspaper items--,further reflections on slavery, sermons, anti-slavery speeches, poems, appeals to the reader of rfunds and moral support in the battle against slavery.