You can blow it up more, if you really want to, but the bigger your balloon is, the more yarn you will need. Don't blow your balloon up too small, or it will look more like an egg (although this will work great for a balloon gourd!). If you blew up your balloon half-way, you will need 21 pieces. If you blew your balloon up more than that, you will need more pieces of yarn. For a traditional pumpkin, use orange yarn. You can also use ivory yarn for a white pumpkin, or even purple or black!  Use plain, medium or worsted-weight yarn. Don't use anything too thin or chunky. Cotton yarn will also work well for this project. The easiest way to do this would be to tie the other end of the yarn to a cabinet handle. You can also tie it to a clothes hanger, then hook the clothes hanger onto something. This will keep your balloon suspended in mid-air and make it easier to work on. You will need 2 ounces (60 milliliters) of white school glue for each yarn pumpkin that you are making. Do not water the glue down, or your pumpkin will be too flimsy after it dries. For a stronger pumpkin, try a decoupage glue (ie: Mod Podge) mixed with a little bit of water. Be sure to use a matte finish if you'd like to preserve the matte texture of your yarn. Use a fork to swirl it around, making sure that it gets evenly soaked. This will get just the right amount of glue off. If you don't do this, the yarn will drip everywhere and make a mess. If you use your fingers, you will take off too much glue, and your pumpkin won't be strong anymore. Hold the balloon steady with one hand, then press the end of the yarn close to the tail with the other. Wrap the yarn around the balloon until you reach the other end. Press down on the end of the yarn to seal it in place. Be sure to change directions every so often. You don't always have to start at the tail end of the balloon. You can even wrap the yarn around its equator for a different effect. Remember to press down on each end of the yarn to seal it. The yarn won't completely cover the balloon. You will get something that looks like a basket or filigree. Even though you squeezed the excess glue out of it, it may still drip. It would be a good idea to place some old rags, a cheap tablecloth, or some old newspapers of magazines under it to protect your floor or counter. It will take about 24 hours for the yarn to dry. The yarn likely won't be rock-solid once it dries; it may still have some flex to it. This is very important. If you don't do this, the balloon will pull the yarn inward as it deflates. Carefully stick your finger through the holes in the wrapped yarn, and run it between the yarn and the balloon, prying the two apart. If you notice any yarn still sticking to the balloon, gently pry it away. Don't let the balloon pull the yarn inward, or your pumpkin will appear dented. Pull the balloon out when you are done and discard it. At this point, you can cut off the tail that you used to hang your pumpkin. You can also leave it on so that you can hang your pumpkin up someplace more visible. Wrap a brown pipe cleaner around your finger or a marker to create a tube. Carefully pull it off.  You can make vines in a similar fashion. Wrap a green pipe cleaner around a pen or pencil, pull it off, then gently pull it apart to create a coil. You can make leaves out of green paper or fabric. In most cases, you can simply slip the end of the pipe cleaner under the yarn. If this is not possible, however, attach it using a hot glue gun. Hot glue the leaves to the vines, then attach the vines to the base of the stem.

Summary:
Blow up a 12-inch (30.48-centimeter) balloon half-full. Cut your yarn into 36-inch (91.44-centimeter) long pieces. Tie one of the yarn pieces to your balloon, then use it to hang your balloon up. Fill a container with glue. Dip your first piece of yarn into the glue. Run the yarn between the tines of the fork, pulling the yarn slightly to the side as you do so. Start wrapping the yarn around your balloon. Continue dipping the yarn into the glue and wrapping it around the balloon. Let the balloon dry. Use your fingers to press the balloon away from the yarn. Pierce the balloon near the tail and let it deflate slowly. Make a stem out of a brown pipe cleaner. Attach the pipe cleaner to the top of the pumpkin.