Tell them in class and tell them on the syllabus. Tell them cheating hurts everyone and that they should not hesitate to inform you if they witness such an act. Select the ones that best suit your style. It is their responsibility to behave honestly, but we continue to have an abundance of cases year after year.
Instructors can promote honesty by making it difficult to cheat.
There are many ways faculty can create an environment that supports academic integrity. Give specific topics for assignments. Keep copies of past papers. If possible, familiarize yourself with each student's writing ability. Make it clear whether or not students are allowed to collaborate on take-home assignments.