Compartmentalizing your plan by developing planning modules allows you to attack the plan in parts, yet still maintain a cohesive plan. I have found that developing an annual plan made up of quarterly targets - thus becoming a rolling quarterly forecast financial model - allows for a cohesive structure along with the nimbleness to react to market conditions. At the end of each quarter, a true-up process to align results to annual targets needs to be re-forecast and adjustments made.
The first step is to identify the key company goals which will be the over-arching direction of the plan. These goals should be focused on three areas: financial, growth initiatives and alignment to the company vision/mission.