The easiest way to do this would be to span a fishing line between the last link-o-loons on both sides of the arch, and then tie the megaloon letters to the fishing line. The megaloons need to be helium filled to make them float. I believe they would hold up the link-o-loons, so you could fill those with air.
Here is a good resource with info and instructions around using link-o-loon balloons.
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I have checked some websites that were recommended by Balloon-Decoration-Guide.com in answers to some of the questions posted here. This was very helpful.
My mom and I are planning on making air-filled swags with the 6 link-o-loons. We have never worked with link-o-loons before. How many days in advance can