Here is an idea about how to teach I wish/If only in approximately two lesson hours. Below is a summary of the lesson but you can reach the detailed lesson plan at the end of the post.
Warm up starts with a powerpoint presentation:
I wish-if only
Second step is a Prezi presentation:
Students try to guess the grammar structure.
Then, they revise the grammar structure with a video including some exercises:
After exercising enough, they form sentences with the structure talking about their present and past regrets. They send these sentences to the following link so that every student will have an opportunity to see others sentences on screen during the class.
At the end, in order to repeat what they have learned, students compete in groups while answering the questions of the socrative quiz. Here it is:
I hope you and your students enjoy the course.
The detailed lesson plan: Lesson plan – Ifonly-Iwish