I had a shed built at my old place, and converted it to a coop myself. It was a typical 8x8 with a normal gable roof (simple peaked roof). On the windward side, I put in two, foot-square vents (that was all that fit), and on the leeward side I opened up the entire gable (triangle shaped top) and put hardware cloth and shade cloth. I'd have loved to put more, but I didn't have very many birds in it for the size of the shed (4-8 birds in a 64 square foot shed). I'd keep the ventilation well above the roosts. I used a drill then a jigsaw to cut the openings for the ventilation in the siding. I used screws with washers to screw in the hardware cloth, after using staples to get it into place easily by myself.
I also had to screw one of the twin/double doors shut to make it secure and fill in some gaps along the bottom with steel wool and hardware cloth for pest exclusion. I had the shed built without the floor, but if yours has a floor I'd put linoleum on it for ease of cleaning, unless you build in poop boards.
To mount the roosts I had to sister 2x4s along the existing framing studs to give me a better way to screw the roosts in. The roosts were 2x4s with the 3.5 surface parallel to the ground, so they had wide perches. They ran the length of the shed. mine were 30 and 36 inches, depending on the types of birds you might want higher or lower. I put blocks near them so the birds could hop on and off more easily.
I built the nest boxes into the wall, but if I did it over again, I'd just put them freestanding so I could redesign them if they were too small, too hot, or not private enough, or if birds preferred to roost on top of them....
I've uploaded the only picture I have.