When I first introduced the passive to my pre-intermediate Cambodian students, their initial reaction was to look at me as if to say, "You're telling me English has another tense? Why, teacher? Why?"
They're definitely warming to it now... I think.
The Oxford textbook that we use tends to jump in at the deep end with grammar points. While the passive may be a breeze for speakers of languages that form it in the same way (eg. French or German), it will understandably take a bit more work for speakers of languages that have no such thing.
To bridge the gap, I made a handout that explains how the passive is formed and what it is used for.
It progresses to some guided practice in the present tense.
Link opens with Google documents, you can then print or download as required