If you are teaching with the deductive method, you write the rule up on the board and then ask them to apply it. If you use the inductive-communicative method, you can take any kind of text, which provides you with a context, and use it. Say you want to teach the present simple and show how the third person has an "s". You can take a simple text describing a person's daily routine. Then you can ask the students to underline all the verbs. Ask them what they notice about the verbs. You will THEN write the rule up on the board; the present simple I .drive........., you .......... he ..............
They tell you that you add an "S" at the end of the present simple. The communicative is more student-centered than teacher centered. After you have let them "infer" the rule, you can get on with the controlled practice that Teacher Joe suggested. This method can be used for any grammatical structure.