Dear Moon Unit,
I suggest you take a text containing examples of the passive. A description of a procedure is one type of text that lends itself to the use of the passive. You might find examples in Headway or English File course books; alternatively, you could write your own text or prepare a series of sentences. If you create your own text or sentences, use vocabulary that the students are familiar with. If you use new vocabulary, you might spend more time in class working on vocabulary and not on the passive.
Do some comprehension work with the students to make sure they understand the text; prepare some questions that they can answer. You should then focus on the sentences containing the passive. You could give students an active sentence and ask them to find a sentence in the text which has the same meaning. For example, give them the phrase “somebody stole his bag yesterday” and they look for the sentence “his bag was stolen yesterday”. Do this with 3 or 4 sentences.
Put the sentences on the board and do some language work. First of all, check students understand the meaning. Ask them who stole the bag; the answer is that we don’t know. For a sentence such as “letters are delivered every day”, ask them who delivers the letters; they might give answers such as “the postman”. Ask them what is the focus or important aspect of the sentence; the answer will be “letters”. (Make sure you choose a text where the focus is on the letters and not the people doing the delivering). Explain that this form is mostly used when the person causing the action is unknown or unimportant (i.e. not the focus).
You should then look at the form of the passive; ask students what they notice about the construction and how it differs from an active sentence. Ask them who is mentioned first in an active sentence: the person causing the action; in a passive sentence the person causing the action is not mentioned or is mentioned at the end of the sentence. Write on the board something like: passive = to be & past participle and show how the order changes between active and passive sentences.