Separate the balloons into groups of three

What do you need? Balloons in any colours you want, string and a strong wire (double it if it is too thin) as long as you want the arch to be.

Start by tying together three balloons of the same colour with a piece of string. Repeat with all the balloons.

String the balloons on the wire

Once the balloons are tied together in as many groups of three as needed to cover the arch, start tying them to the wire using another piece of string. The idea is to alternate colours in the same order.

Then you just have to decide where to put the balloon arch and tie it well into place. Looks cool, right?