Well ... I do something similar to this ... sometimes
1. Take a pen and a piece of paper (perhaps several ), sit down (away from the computer), start thinking on WHAT you want it to do . Start sketching up what parts you need, a basic layout how stuff should be viewed, and perhaps even start sketching on how you want to save the setuff (ie. basic database tables, what kind of values you want to store, what tables that communicate with each others etc.)
2. Start the computer, and make a test about the layout, is it what you want it to be (just use phony values and stuff, dreamweaver etc is usually quite fast to make some basic design... just to test ideas). It's usually now one will find out if the ideas one had in the beginning might work or not, if something has been forgotten etc.. When the layout etc is ok enought, I go on to the next step (Note: the layout will probably be different in the last version either way, but it will help to structure the rest of the work to have something to aim for)
3. Start writing basic functions, like database connections, login system etc. (and structure how stuff should be saved on disk), start creating databases etc. (for me they go through quite a few revisions, always small columns added now and then ... so I always write queries with SELECTS really defining what column and in what order I take them, as othervise it'll be a bitch to update database and scripts)
4. Well when basic login/database stuff etc is working, time to move on to actually starting to write the stuff needed. Page generation, upload/download things, etc.
5 - 20... New Code, bughunting, rewrites
21. Final touches
This is how I usually work on larger projects ... I'm mostly in steps 5-20