I don't know anything at all about doing what you're suggesting. But if it was me, and I really wasn't that fussy about it being fancy, I'd just try it. If it doesn't work, you haven't really hurt anything and you could always go back and pour concrete later if it was necessary. But that's just what I would do. You might want to wait for someone that actually knows something to answer your question!

You could use the flagstone as steppers, but build a small foundation under each one with 2 cinder/cement blocks side by side and back filled with soil. I would make a riser in front of it to retain the soil from washing onto your step. I built a stairway for a client of mine using this method.
Take care,
marg

We are building a flag stone walk that is still under construction, but we created a gradually sloping walk with flagstone closely arranged as if you would be doing a patio. The stone is set on the compact ground and then I washed a bagged concrete mix into the gaps. After one winter, the steps have hardly settled. So with that said, setting the stonge without much prep on compact ground is of little concern. And as it was said above, if it doesn't work, rework the idea. For a longer walk, creating larger spaces is helpful as the flag can be pretty expensive around her. In that case, the flag just takes the place of the "stepping stones" you mention. Just buy them thick enough so they don't crack.
MJH

Rhaine, I had the very same thing done by a garden designer.
It was over 2 years ago, but I watched alot of the construction on my slope. They used a sodcutter to remove all the grass in a wide strip. Then dug and made shelves and laid small gravel on the shelves. Then fit the thick (3in or more) flagstones onto the flat shelves and shoved and packed until they were solid. I ended up with 4 wide but shallow steps up, then a four ft paved 'landing', then 3 more steps up, etc. to the top.

To finish off, they dug in edging material about a foot out from the flagstones and covered the bare dirt with mulch.
Between the stones some of the bottom peagravel showed thru, so they covered that with a layer of pretty round and multicolored stones. (about ½ to 1 inch in diameter)

My walkway is four to five feet across, and about 20 feet long from top to bottom of the slope. The flagstones are really, really large. Long and narrow strips fit under the tread part, and are sort of like the riser of a regular stairway.

They started at the bottom, laid one large stone, then the riser, then the next stone overhanging the riser just slightly.

It is a beautiful piece of work. I hope you go for it.
Pondy