In Julius Caesar's Commentaries, he connects the burning of a wicker man to the Druid practice of human sacrifice -- essentially, the wicker man was a cage in which a real person was placed. Fortunately, that practice died out with the Druids, but many people still like the idea of creating a man from the detritus of the garden at the time of harvest's end. In some Pagan and Wiccan paths, this man is known as the King of Winter, and he can be created in an altar-top size to watch over your home throughout the chilly months.
This is actually one of the easiest and most primitive projects you can do. You can incorporate it into your Samhain rituals, or make one any time. You'll need two bundles of leftover plants out of your garden (if you don't have a garden, it's perfectly fine to gather some plants at the side of the road) and some string. If you're using plants from your garden, feel free to mix and match different branches. The straw man in the photo is made from hyssop, rosemary, and stevia. Make sure one bundle of plants is slightly thicker than the other.
With a long piece of string, tie the fatter bundle together about one fourth of the way from the top. This end becomes the head.
Separate the bundle a little bit, and slide the thinner bundle of weeds through the center. These will be the arms. Use the string and wrap in a criss-cross shape around the body to hold the arms in place. Tie it off to keep it tight, but don't cut the string.
Finally, spread the lower part of the fatter bundle apart, forming two halves as the legs. Bring the string down and wrap around the "thighs" to keep the legs in place. If your branches seem like they're too fluffy, tie a small length of string in place around the wrists and ankles; as the greenery dries it won't stick out as much.
This is a very basic design, and you can either leave your straw man as rustic as you like or pretty him up a bit, it's entirely up to you. Save him until Spring, and then burn him as part of your Beltane celebrations.