I have a lot of experience teaching idioms to different levels of students.
I recommend you majorly use pictures to make the process easier than you can even imagine. I personally use 8-12 steps to teach a group of idioms.
1. Show them the pictures and ask random questions about what they know about the pictures and in this case pictures of the animals. ( Emphasize on the most common characteristics of those animals)
2. Tell them a story using the idioms you intend to teach.
3. Write the idioms on the board as you are telling the story.
4. Ask them to guess what the new idioms mean according to the context.
You could have a group work here if possible.
5. Tell them the meaning of the idioms.
6. Give them extra examples using the new idioms ( in context if possible )
You could ask them to create their own examples.
7. Re-tell the story focusing on the new idioms.
8. Give then a group work activity to write very short stories using the new idioms ( you could make a homework out of this step if you are pressed with time)
9. Ask them to read their short stories to keep them motivated.
10. Ask the other students to summarize thier partners' stories.
Using OHD seems very helpful if you have access to one otherwise you should rely on your artistic talent to draw pictures for them. They will be amused. This is a rough method that you could surely make much more interesting by adding your own steps by considering the class situation.
English Teacher Trainer