I've had a lot of questions lately when I post my favorite fermented feed guide on BYC.
http://tikktok.wordpress.com/2014/04/13/fermented-feed-faq/
I don't want to hijack that thread, so I'm going to quote it here and answer what I can.
I fill the container a little less than half way, then Fill it past the water level a bit. Stir it through, it should be pretty hard to stir by the time you are finished.
That's when starting a new bucket. If you already have some fermented food, you add a scoop or two to the water and stir it up before adding the dry and incorporating that to a uniform consistency.
Others have said an adult chicken eats from 1/2-1 cup per day. It varies dependent on forage available as well as other things included in the diet, such as scraps.
If they don't have any left the next day, feed more this time. If they have a lot left over, feed less.
As with mixing it, determining how much to feed your flock is too dependent on too many variables for any one answer to be correct.
p.s. I stir daily when starting a new ferment, but not after the first time. If it has been pre treated with fermented media, I don't stir, but I go through it pretty quickly. I keep it covered slightly to keep humidity in. If the sides get food on them, it can cause mold from not being in contact with the ferment culture and drying out too much.