Hi Samantha
Just say "to be " is a special verb you have to learn it by heart
then you can introduce regular and irregular verbs
You can find very good worksheets explaining the Simple Past with
a drawing + verb + ed
(showing a person
= I , you ....)
then you can find the same for the negative form and the interrogative form
you can use the same for the irregular verb
explain that the form of the irregular verbs have to be learnt by heart, too
I did it this week with two classes and it worked quite well
after having explained the grammar practice with many games to learn the new forms
go step by step
Good luck and MERRY CHRISTMAS
Swissprof