When installing shingles, you need to put down roofing felt. The roofing felt is what dries in your shed/coop, then you need to install the shingles. The shingles protect the roofing felt. Start at the bottom and work your way up. Keep in mind the first row of shingles is put on upside down (the tar strip up), then lay the next row on top of the upside down shingles at the bottom edge. This will protect the roof's edge when your completed.
If you never installed shingles, it's not that hard. You just really need to do it correct the first time.
For my coop, my neighbor was throwing away an above ground swimming pool that is 4' tall, so I took it and cut 3 pieces 12' long and overlapped it about 1.5'-2' at each joint. I was in the coop while it was pouring down rain out and it was nice and dry inside.
Nothing like a free roof. All I had to purchase was the 2x4's for the rafters.
Hope this helps