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You will need 16 15-foot (4.5-meter) ash saplings that are about 2 inches (5.08 centimeters) thick for the frame. You will also need at least 12 ash or basswood saplings that are 15 feet (4.5 meters) tall and 1 inch (2.54 centimeters) thick for the horizontal hoops. The saplings need to be straight as possible.  Ash and basswood are the most traditional, but you can use other hardwoods, such as: cedar, chestnut, elm, or hickory.  If you are making a model, use thin, bendy twigs instead. You can also use wire instead. Florist's wire coated in brown paper would work well. Start by trimming off any bark and branches. This will help reduce wood rot and help the wigwam last longer. Next, cut the ends of the saplings into points. This will make it easier to insert them into the holes.  Make your poles extra-strong by hardening the pointy ends in a fire. This will also help them last longer once you drive them into the soil.  If the saplings aren't flexible enough, soak them in a lake or river for at least two days first. Be sure to tie them down so that they don't float away. Gather 50 bushels of cattail leaves. A bushel is a bundle that is thick enough to wrap your arms around. Separate the cattails into 2-inch (5.08-centimeter) thick bundles. Secure each bundle at one end with some string. Set them out in the sun to dry.  If you don't have access to cattails, you can use heavy canvas, which was used to cover wigwams from the late 1700s onwards. You will need 12 to 14 sheets that are 5 by 10 feet (1.5 by 3.5 meters). If you want to be more traditional, you can sew cattail leaves into large mats. Plan on using 12 to 14 mats, each 5 by 10 feet (1.5 by 3.5 meters).  If you are making a model, you could use pieces of grass, raffia, or straw instead. You can also use pieces of cotton, linen, or canvas fabric. Gather 7 to 10 sheets of birch bark, about 3 feet (cc meter) square. Use a hot nail or wood burner to make holes along the top edge of each birch bark sheet.  You can also use elm bark, or weave mats out of cattails.  If you are making a model, you can use white paper instead. For a more realistic effect, go over it lightly in a dry brush dipped in brown paint to make the streaks.

Summary:
Gather some ash and basswood saplings. Prepare the saplings for the frame's poles. Prepare cattail leaves for the thatching. Prepare the birch bark sheets for the covering.