When introducing a new assignment, be sure you've carefully reviewed it yourself beforehand. Highlight key places where you'll want to elaborate with examples or explanation. Also, anticipate any questions or confusions students may have.
Plan to check for understanding by asking students to summarize or interpret certain aspects of the assignment. For example, have them analyze the writing situation by asking, "How does this compare to the essay you just finished? Who is your new audience? How will you need to shape your writing to meet the needs of this audience?"
If a student raises a question about a concept or an assignment that you don't have an answer for, simply tell them you'll get back to them next class.