Words of Wisdom
There are always three speeches, for every one you actually gave. The one you practiced, the one you gave, and the one you wish you gave.
Lecturing is not the preferred form of instruction in writing. Hillocks, a writing researcher, explains that the epistemological basis for using lecture as a form of teaching is a belief that "teaching is tantamount to telling" (page 18 in Ways of Thinking, Ways of Teaching, NY: Teachers College Press, 1999). However, writing, being an activity that requires practice and that must "pass through the filters of past experience" (19), is best learned by doing. Although lecturing can be used in teaching writing, Hillocks, who conducted a meta-analysis of studies of the effectiveness of various forms of writing instruction, warns that "recent research strongly indicates that such teaching is largely ineffective" (134).
However, limited lecturing, if accompanied by discussion and the opportunity to practice, can be effective, especially for large classes that break later into more active discussion sections. As far back as Cicero, rhetoricians believed that students learned oratory from precept, practice, and talent. Talent, of course, was left to nature, but, as master teacher Quintilian pointed out, the rhetoric teacher could provide practice and could teach precepts (in other words, rules and conventions). Likewise, you can explain the conventional discourse practices in your discipline and explain the specifics of documents you wish students to produce.
Topics suitable for short lecture include:
the research process
the composing process
review of basic grammar and punctuation
documentation conventions for a specific style (MLA, APA, IEEE, CSE, etc.)
the parts, usual content, audience for, and purpose of a given document type
acceptable arguments, including fallacies and refutations
In each case, students benefit from viewing example documents or models and from discussing those models with a more experienced writer. As an example, a lecture on a memo of transmittal for a report would start with viewing a sample memo that satisfies your requirements. After defining the memo of transmittal, you would discuss its audience and purpose, its parts and their possible arrangements. You would stress adapting the particular memo to the situation. You might end by showing examples of inadequate memos to generate some discussion of what sorts of improvement they need. However, be careful not to use negative examples written by students. Just as effectively, you could end by showing good examples and ask students to point out variations among them.