After discussing Form Criticism and Source Criticism, Redaction Criticism is really quite easy to deal with. Redaction is simply another term for editing. It is the study of how an editor works the sources he has into a final document, the document that we would commonly refer to as the autograph. Again, it is important to remember that there is no necessity to assume that the final copy of a document as we have it in scripture went through a stage of redaction. The epistles of the New Testament are good examples of documents that would require either no redaction, or would only involve minimal redaction.
In the Parable of the Sower, elements introduced by the redactor–in this case the gospel writer–include the setting of the parable, the place where Jesus is said to have related the parable, and possibly the interpretation. It is commonly thought that Jesus did not include interpretations with his parables. I would suggest that the form of the parables gives some support to this theory. They are best suited to use in stimulating thinking, without long explanations. But in my opinion that doesn’t mean that Jesus and his disciples never discussed the meaning. Thus multiple applications of a parable could legitimately have arisen during the lifetime of Jesus. The placement of all the interpretations, and some of the interpretations themselves then would be redactional elements. It is unlikely that Jesus immediately followed parable with interpretation in his normal style of teaching.
Another example comes from Matthew 5:3 and Luke 6:20. Luke 6 reads “Blessed are the poor . . . ” but Matthew reads “Blessed are the poor in spirit . . .” Because of the form of this saying in the Gospel of Thomas, v. 54, which also reads just “poor” it is likely that Luke’s is the more common, and possibly more original form of the saying. Thus the addition of “in spirit” by Matthew (as redactor) indicates something about his theology and his intention in writing his gospel. (Note that use of the Gospel of Thomas is controversial here, because there is some debate on its date and whether it constitutes an independent witness to the sayings of Jesus.)
To look at an example of sources and redaction, see my essays The Two Flood Stories and Genesis Creation Stories – Form, Structure, and Relationship.