Old hands already know this, new people to GtD will eventually figure it out. The process that is GtD can, and should be customised to an individuals personal circumstances following the principles that David has detailed in his book, GtD is not a one size fits all solution.
Having said that it might be helpful for new people to GtD to see the process, mapped into a decision tree and to that end I attach 'myGtD process flow'.
Just remember this is where I am at with the process and it may / may not work for you, so it's offered on the basis that it's a suggestion not a proscription
Its done in Visio 2003 but it is attached as a .pdf. If you would like to play with it please send me a private message and I will email you the .vsd version.
Comments welcome, Enjoy!