1. Helium fill 2 balloons and get them both the same size by holding them side by side. Then tie them together.
2. Repeat step 1 and twist the 2 sets of balloons together into a cluster of 4.
3. Twist 1 balloon around your line (or twist the line around 1 balloon in the cluster).
4. Repeat all of the above until your arch is the required length.
5. Attached weights and knock the arch into shape.
The garland of balloons will naturally form an arch shape because the whole thing is floating. If you use 12" the arch will last approximately 20 hours. It's always best to build on the day of your event.
If you want to build an arch using air you'll need a frame and sturdy bases.