How to: cut some long pieces of grapevine. Use one to make the size circle you want for your wreath, and secure with some twine or wire. Then simply tuck the end of the next vine into that and start wrapping one vine around the other all the way around. When you run out, stick another one in and keep going.
When it looks done, you can stick marigolds or fall foliage pieces through it and they should stay. Have fun!