When providing the sample, you might conclude that the excerpt needs context for the readers to know what you are writing about. It is usually the case.
An introduction or an overview sets the scene and explains the subject matter of the writing sample. It helps understand everything important and should help make sure that the reader does not miss any important information. It should be placed at the beginning.
Apart from explaining the context, the introduction should also inform the reader about whether any confidential information has been redacted or altered and whether someone collaborated on the sample or went over it to polish it.
Readers Have To Know the Writing Is a Sample
Another important thing to keep in mind is that the readers should always know that you provide them with an excerpt and not a complete document. If this is not clear, it can cause unnecessary problems. The best way to do this is by including a header (or a footer) on every page with a clarification of what the document is. It could be, for example, [YOUR NAME] - Writing Sample. If confidential information has been redacted from the document, also include the word redacted in the footer/header.