To start, go out in your woods and cut down some tall, straight trees, with about an 8" diameter. Trim them all up, and then...
You will need:
12' posts X2
12' poles X3
8' posts X4
and a bunch of smaller stuff I'll send you back in the woods for later, lol. Give yourself an extra foot or so to level them off later as well.
Measure 12' forward from each corner and mark the front corner post spots. Then measure 12' between them. You'll need to adjust to square it all up. Once you have it all square, start digging. Drop an 8' post in each corner.
Cut your first corner post off flat at 4' above ground. Cut the matching front post off flat at a level height. Then do the same from the original corner post to the opposite back corner. Then that corner to the opposite front corner. Check the level with the first front corner before cutting.
Drop your 12' center posts into their holes. Measure one at 8' above ground, cut off flat. level the other, and cut off flat.
Pop the 1x3 off with a crow bar, and roll the plastic up on it, and nail to the roof to raise the sides.
You can build a solid door if you want. We did, to keep critters out. The door plastic is just a strip nailed above the door frame, with a pole stapled on the bottom. I put in a couple of spikes at the top of the door frame to rest the pole on when rolled up.We also added a couple of one by threes on top where the seam of the plastic over lapped. A pole would be fine as well, just something to staple the loose edge of the plastic to.
Clear as mud? Good luck to you. I hope this helps!