Kelly, I'd love to see your Setup Guide when you complete it. I love Todoist and I love GTD, I've used Todoist for about a year now, but I have no brilliant ways of using either projects or next actions.
My projects are ...well, projects (..!) and I simply have a Todoist label for Next Actions. I can give any task that label, even a task that isn't currently part of a project. I can check my current list of Next Actions at any time by clicking on that label. But to me, that is far from ideal.
And the only thing I don't like about Todoist, is no automatic identifying of next actions, such as how GTDNext does it. In GTDNext, the first item in any project is automatically idenfitied as your Next Action, so that your 'Next Action' display is uncluttered - it has one and only one task. To me, it meets David Allen's criteria of being able to focus on your Next Action. And if you click 'Complete' on that task, the next task in the project list automatically pops up - again, just one task, in an uncluttered display. A very good system. I've even asked the Todoist support team if they have any future plans for automating a Next Action similar to GTDNext's method, but they don't seem to have any plans for that. Following the way GTDNext does it would seem to me to be the ideal way to do it.
Absent any automatic process like GTDNext, it seems to me anything Todoist tries to do with Next Actions will be clunky and clumsy.