I created this activity with my JTE to help my JHS 3rd graders with "if I were... I would...".
First, use the powerpoint to give a short talk using a lot of visuals. This talk uses second conditional grammar a lot in a way that students should understand easily.
After the examples, turn the question to the students as a class. Ask them "If you were a teacher, what would you do?". Have them talk in pairs or as a class (for smaller sized classes) for 2 or 3 minutes.
Hand out the worksheet. The worksheet has more examples not covered in the powerpoint. Give students a few minutes to complete some of the blanks, then take volunteers to tell the class about their alternate realities. This part of class is always interesting because some students make funny sentences. This activity doesn't take long and my JTE preferred to use it at the end of class as a good review and practice for the grammar.
Hope this is helpful!