A balloon arch is a festive decoration and a handy marker for guests looking for a party. Since helium balloons float, this arch practically builds itself. All you really need to do to make a rainbow balloon arch is to get the balloons in order and connect them to each other and the ground.
Buy your balloons and helium. To make a rainbow balloon arch, you'll need balloons of many different colours. You want the balloons to be as fresh as possible and you'll need quite a few, so your best bet is to get a tank of helium and fill the balloons yourself. This way you can also decide as you go how many balloons you will need in each colour.
Tie a very long piece of yarn securely to a heavy anchor item like a rock or very thick book, or a stable location like a railing. Thread the other end of the yarn onto the needle.
Thread this yarn onto the needle and begin adding balloons. Poke through the extra bit of balloon outside the knot, and push the balloon all the way to the other end of the yarn. Keep adding balloons, gently pushing them tight after each addition. To make a rainbow balloon arch, add balloons in rainbow order.
Check the length of your chain occasionally by holding the unfinished end near the ground and to see the height of the arch. Add balloons until you are satisfied with the size of the arch.
Securely attach the unfinished end of the yarn to your second anchor item. The buoyancy of the balloons should pull them up into an arch shape naturally.
For more security, thread two pieces of yarn through the balloons simultaneously. If one breaks, the second may hold. You can also tie a knot around the base of each balloon as you go, so that even if the yarn breaks the balloons won't all fly away.