How can one teach apostrophes effectively?

First I must know the level but I tell you what can I do for ESL classes

I write on the board

Boy toy       boys toys         children home         frogs legs             a door key           Cairo population

  two weeks vacation

  and then ask students to insert apostrophs to differentiate between singular , plural , and items that don't take apostroph . 

you only give them all types of adding "s" and tell them which on of them need apostrophe and they diiferentiate between them

Must first learn well