Start with a very simple domain model as a strawman to help you understand and iron out the first complexities of the problem you are trying to solve. If you can explain the structure of the idea it will make it much easier. Then as Dian says start to break that model down using user stories or feature lists. This will help take that domain model to the next level.
Once you have that in place, focus on the problems that don't have easy answers. This should be obvious now that you have a model in place as they will be the parts you are struggling to describe. If you sort these first you won't get caught at the end with an unfixable problem :-)
Hope that helps...
Thanks, this will help me begin the project without distraction.
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