Structure your design plan by laying out the goals of your multimedia text by discussing your audience, context, and purpose (statement of purpose). Then turn to how your choices of medium, arrangement, and strategies accomplish the goals you have established. Look at pages 52-54 of Compose, Design, Advocate excerpt 2 for information and examples.
Don't use "I" at all if possible. Don't talk about your process producing the text; talk about what the text does and how it works.
Remember that for the final draft of your design plan, your multimedia text should be in a finished state. There should not be statements in your final design plan about maybe doing this or that. At this point, your multimedia text is a finished product. You should only talk about what it does and how it works.
Your text should be given to me in its intended final format. If, for example, you are making a poster, you must give me the actual poster, not a smaller version printed on a piece of paper or a digital file.