Vitamin C speeds up development, it is in itself, at least to a certain degree, a developing agent. Some Apug'ers (titrisol and friends) did some tests 7 or 8 years ago with different salts to reduce stain and get better shadow detail. Ascorbic acid (aka pure vitamin-C) evidently did the job, combining with some of the sodium carbonate (washing soda) to form a salt called sodium ascorbate. I mix the soda and vitamin-c first, let settle for a few minutes and then add coffee last minimize the exposure to the dreaded caffenol smell (not that bad really). You could add a lesser amount of sodium ascorbate instead of vitamin-c, but its more difficult to get hold of. There are two approaches to Caffenol-C type recipes, those with little vitamin-c and those with considerably more. What you end up doing is up to you, its a matter of taste really.
Since I´m in no way an expert, I´ve relied on the findings of others, and use the Caffenol-C-H and C-M recipes from the caffenol.blogspot.com site mentioned by John. The C-H recipe has a potassium bromide buffer for high speed films (400 ISO and up), the idea is that it will reduce base fog. Slower films (IS0 100 and mot likely up to 200) can do without. With either recipe 15 minutes or so @ 68F is the recommended starting point, for films developed at box speed or pushed one stop. I have however seen evidence that FP4 may well do with less than that, 12 minutes or so in Caffenol-C-M (no bromide buffer).
For my first ever Caffenol developed film I bracketed every shot -2/-1/0/+1/+2 to get an idea of what would need to be done next time around. And sure enough, next film I more or less nailed it.
I too use water stop bath. The film will continue to develop somewhat until the end of the stop bath and before being dunked in the fixer, but I wouldn´t worry about it. I use TMAX fixer, but that's just because it was what was handy.
