I'm preparing dinner.
(You're mixing ingredients, boiling, baking, frying, etc.)
I'm preparing for dinner.
(You're washing your hands. You're going to eat very soon.)
I'm preparing an exam.
(You're writing the questions that someone will answer when they sit the exam.)
I'm preparing for an exam.
(You're reading and thinking about questions that might be included in an exam that you are about to take.)
Returning to our opening example, I think you need:
Write sentences about the picture below using prepare and prepare for.