This is probably the easiest step to teach, because you will be met with little to no resistance from your students since it is the step with which they are most familiar. They are accustomed to writing rough drafts. They have been writing rough drafts since they were in kindergarten; the problem is that many of them have only been writing rough drafts. They have not been pre-writing, revising, and editing. That is where your challenges will lie.
Still, use this time of relative ease to build confidence in your students and their trust in you. Compliment their drafts, make suggestions here and there, but be positive. You will want them going into revision and editing fresh and willing to work for you.
When students start their rough drafts, make sure they use their pre-writes to guide their writing. Require their pre-writes to be on their desks, side-by-side, with their rough drafts. Otherwise, you have all wasted valuable time and effort. Point out to them again and again how much of the work is already finished because they took the time to complete a pre-write. This will help your students “buy in" to the writing process. When they see their time working on the pre-write was not wasted, that it is actually making their lives a little easier, they will start to (grudgingly) develop an appreciation of the process.
Since students have already decided much of the actual information they are going to convey (in their pre-writes) their minds are freed up to look at the more technical aspects of their writing. Introduce them to words that help transition from idea to idea. For example, the words additionally, furthermore, and moreover can be used in place of also. Prepositions can help them to describe places in more detail. You can obtain lists of these types of words on the web. The rough draft is the perfect time for students to go ahead and insert these words into their writing.
Of course, remind students that if they want to add something not included in their pre-write or leave out something, they are perfectly free to do so. Pre-writes are simply guides to help write the rough drafts.
It is up to you to determine how to grade the rough draft. Some teachers choose to incorporate it as part of the final draft grade (for example, 15% of the grade) while others choose to assign it a stand-alone grade. Whatever you do, give students credit for doing it. They have to see that it is important enough to warrant a grade, or they will either refuse to complete it or give poor effort.
Keep in mind that rough drafts are not where you look for perfection. You are simply making sure the pre-write was followed, the amount of work required was finished, and whatever else you assigned was completed.