standard rules apply
1 define your high level business aims
2.define your high level technical requirements
3.drill down each of the above into the greatest details you can manage.
4.mock up screens that fulfil your requirements in 3
5. extract the entity model from the work in 3.
6. construct a normalised data model from the entity model.
7. assuming you are coding in oo start building your classes to map the entity model
8. define the entry points to your app and code these either as pages (bad idea) or as functional entry points in a switchboard (better).
9. consider whether you need any pervasive functional overlays like user management, granular access control, authentication layers etc. work these up to beta stage before going too far in to the meat of delivering your business rules. so parallel with 8.
10. flesh everything out with the middle ware layer addressing your business rules.
11. parallel with whatever stage you want have a designer start improving your page mock-ups and when settled start coding them.
12 alpha test as you go.
13 beta test at milestones. allow 35% of project time for defining requirements 40% of time for testing and the remaining 25% for coding. then add 40% extra for scope creep and unforeseen delays.
there are some user management layers Pre built. likewise some PayPal integration classes that you can leverage. but don't use anything prebuilt until you fully grok the code otherwise you can't support your own app. if in doubt wrote it yourself. if you're still in doubt hand the whole thing to someone else and negotiate an excellent support contract before you start.
