Teaching students about behaving appropriately online, I have learnt, starts with students understanding that messages sent online can be interpreted differently than they are face to face. To explore this, an idea for a lesson would be to display words or short sentences to students and see how many ways they can say the words using different facial expressions, body positions and tones of voice. Students can then discuss how, one sentence can be interpreted many different ways, especially if the context of the conversation is not clear. Therefore, when online it is important to be very clear of your intended meaning.
Students can read the comic strip https://www.unodc.org/documents/e4j/FunCorner/COMIC_03_web_CUN.pdf to help them with their understanding. This comic can lead to further discussions as a class, leading to students to identify clear steps that need to be taken to ensure that people are respectful online and whether this is similar to or different to offline interactions.