A clear and do-able plan is vital.
First you need a single page plan that sets out the headlines. See the template in the resources file.
Secondly, a good plan should also have milestones clearly defined as well as showing when you will achieve the end goal.
There are a number of different formats you can use to write your plan, but perhaps the most popular (especially for complex plans) is a Gantt chart. You could also use an Excel spread sheet, MindJet, Visio or even a table in Word.
Find one that suits you but the most important thing it to write it down! Once you have your plan (draft, final or otherwise), everyone needs to be very clear about what they have to achieve - be that as an individual or as a team.