Making a trellis for pumpkins and other vine fruits and vegetables has several benefits. It keeps the pumpkins off the ground, so they don't rot or mold as easily, and they enable a gardener to grow plants that ordinarily would take up lots of space in a much smaller area. Trellising pumpkins is different than trellising tomatoes or berries on a few important points.
Build your trellis before planting, so that you can properly space the plants along it. Set wooden stakes or fence posts upright in the ground about every foot or so in a line where you want the trellis.
Nail wire or fencing between the stakes. Chicken wire will work for smaller varieties of pumpkins, but for heavier vines, try wire grid fencing suitable for animal pens.
Space plants about every 3 feet along the trellis. You can put them on both sides of the trellis. Plant in the spring, when the last frost danger is past.
Train the vines as they grow. Even climbing varieties need some guidance, and ground varieties will need lots of training. Every 6 inches of growth or so, use string, twine or strips of cloth to secure the vines to the trellis where you want them. Tie them loosely so as not to bruise young plants or cause injuries.
As the pumpkins grow, you can make slings for the larger vegetables to help support their weight. Cut a length of rag or old pantyhose and tie it to the trellis on either side of the pumpkin, so that it's holding the bottom of the vegetable.