I have one of these, not used it in a few years but it is fun !
I usually load 1 bag of Portland cement, 5 pails of gravel and a pail of water (to start) add water as necessary to get a good slurry, mix for a bout 5 minutes and head to the site. By pail, I mean the size of the plastic pails sold at Home Depot or like those I find on the side of the road from plaster mix). A few words of advice:
Concrete is HEAVY; Make sure you have enough front weight. I have a decent sized loader bucket and keep it full of gravel or racks to keep the front tires on the ground. And hitting a few bimps will still lift the front tires. That means you may slosh some wet cement out of the mixer at an inappropriate spot. (like on your brand new truck).
Get the hydraulic pour option. When she starts to go, you want to be able to control it. This also means you won't want to dump it all in one spot.
Throw some rocks in afterwards to clean out all the remaining concrete spew. Because you are focusing on the slab, you might forget about the mixer bowl until its started to harden up.
I wind up with a slab about 4' by 4' by 4" or so. I used 2 by 4s for forms to make steps, sidewalk pads, and horse standing areas. (Rake it rough and it will trim their feet nicely if you place their feeder on it).
This mixture us a strong as you would ever hope it to be. I didn't even bother with re-rod. Drove on it with heavy machinery and never cracked.
Tie the dogs up until its dry. My 2 previous Labs left their imprints on it and 15 years later I still get to say hello to them....