Drawing from the information you developed in your user and task analyses, design several different mock-ups, or "lo-fidelity prototypes," of your document. The reason for prototyping is so that you can consider an overall document design strategy without committing too many resources to making polished drafts. The prototype will also provide the plan to guide your group as you put together a final document.
The prototype should be a sketch or mock-up of the overall document design, showing approximations of the final illustrations and text. Although you can do this digitally, you may find it faster and more effective to use such "low technologies" as pencil, paper, scissors, tape, and a photocopier.
After you have produced a prototype and all the members of your group are satisfied with the overall design, you can move on to making a more complete draft of your instructions document.