These balloons should be made at least 2-3 days in advance, so the fondant has adequate time to harden. Different types of fondant harden at different rates, so it's a good idea to give yourself as much time as possible and make these well in advance of when you will need they. They last indefinitely if stored between sheets of waxed paper in a low-humidity, temperate environment.
WireI used 20 gauge stem wire from the floral section of a craft store
Dust your work surface with powdered sugar. If you have plastic food-safe gloves, now is a good time to wear them, so that your hands won't get food coloring on them.
Divide the fondant into the number of colors that you desire. Press one piece into a disc, and put a few drops of gel food coloring in the center. Fold the sides over the coloring, so you have a fondant ball, and knead it between your hands. Soon you'll start seeing streaks of color come through the fondant. Continue to knead until the fondant is a uniform color. Add more food coloring if necessary, until you get a bright yellow. If at any time the fondant gets too sticky, dust your hands with powdered sugar. Repeat with the rest of the fondant balls until you have all the colors you want. Wrap the fondant you're not using in plastic wrap.
Roll out a ball of fondant until it's about 1/8-inch thick. Cut out shapes with your cookie cutters. Snip the wire into the lengths you desire—remember, you'll want to give yourself plenty of room to stick them deep into the cake for stability!
Gently poke a wire into the side of a balloon, taking care not to angle it so it breaks through the fondant. Stick it deep into the balloon. Place the wired balloon on a baking sheet very lightly dusted with powdered sugar. Repeat until you have all the balloons you need, making a few extras in case of breakage.
Let the balloons sit at room temperature so the fondant hardens, for at least 2-3 days (longer is great!).