I built a coop for my chickens, and I'm working on a duck hut now. Honestly, it's never "simple" but there were ways to simplify it so to speak.
You will need to cover the plywood with something. If you can get your hands on free/cheap shingles you can go with that. If not, but you can find some scrap metal that's great choice. You can then tar over the roof to secure any holes/leaks (but you'd be better off going with the grey roofing tar than the black).
Let me see if I can explain how I cemented mine. I had 4 corner posts to cement. Then I needed supports for front wall and rear wall (we ran supports 2 feet out from each corner post for front and rear walls). I cemented each wall support in too. So I had a total of 8 holes dug about 2 ft apart for the front and for the rear (picture a line, mark 2ft 4ft 6ft and 8ft). I built the rest of my supports onto that after it dried. So I put all my wall framing supports up after the cement dried. Then I had ceiling supports to put in, then window supports.
After all of my supports were put in then I had to put up walls and roof. I have a roof that slants from south to north so water won't stand. I used metal from a dilapidated camper to make my walls and sides. I used windows (2) from the dilapidated camper for ventilation on top. I have ventilation on the bottom too. Then for my door I used the door from the camper too. Basically, I recycled all the materials for the coop except for any and all framing work. :) I'm in NC so typically the weather doesn't get too crazy (as you can see in the pic, our rare snow did happen, not a common thing here though).
Here's a pic of my coop during the winter. I had tossed a tarp on the top to help with a leak at the time on the roof (I still need to tar, but I have since fixed the leak).