As I understand it, a funnel is a narrowing of cover that force the deer through a relatively small area. For example, two large wooded areas could be connected with a narrow strip of woods. The deer would most likely travel through this strip. So you would then look for their most used trail. Some hunters create their own funnels on their own land by partially cutting trees to create obstructions (hinge cutting). See the page at http://www.qdma.com/articles/bottled-bucks for some aerial photos.