I've made half a dozen or so very detailed models for varying start-ups and therefore have gone through the process a quite a few times.
1) Please make sure you understand accounting/finance (Income, balance sheets, cash flow, inventory, etc).
2) Who is your target: this determines how detailed, modifiable, and annotated the model should be.
3) Don't worry about the assumptions at first, just get best guesses and build it. Then when done hunt through available financial models to find comparable values. (Have all assumptions on 1-2 pages/sheets and nothing else, I usually have 1 for revenue assumptions and 1 for cost assumptions)
4) Make sales targets/growth easy to modify (in a separate table/sheet) so a statistical analysis can be completed easily if needed or different revenue models can be tested.
5) After you've made the model check your calculations by hand and make sure the values make 'sense'.