I would start by reviewing present tense of be.
You could have pictures of two people (maybe an elderly man and an elderly woman). You could emphasize that this is now.
You could elicit information about the people "He is old". "He is a grandfather". "She is old", etc.
You could then make an area pointing to the left of the board, and draw the date (50 years earlier or something). You could then ask them for sentences about the people then. If they use present tense (for example, "he is young", you could point to the picture and say "is"? That will create the need for the target structure/focus. Then you give them the language they need, and let them generate a new list about the people in the past using was/were.
I hope that makes sense!