Large Mylar balloons -- the type designed for use with helium -- often have a self-sealing neck that holds the helium inside the balloon. Smaller versions inflated with air require heat sealing with a Mylar sealer, hair iron or food storage bag sealer.
Plug in the heat sealer and select a low-heat setting.
Inflate the balloon with a balloon air pump, pinching the neck closed between your fingers.
Insert the neck of the balloon between the jaws of the heat sealer down near the end of the neck.
Squeeze the heat sealer's jaws closed -- or press the lever to close the jaws depending upon the model. Hold the jaws or lever closed for several seconds after the heat light turns off.
Release the jaws of the heat sealer and pull out the balloon.
Double-seal the balloon for a better chance of keeping the air in it for a long time. Seal the balloon near the end of the neck, then again slightly higher up the neck, closer to the body of the balloon.
If the heat sealer doesn't quite seal the balloon at the lowest heat setting, turn the heat up slightly. Use the lowest heat setting possible to complete a seal.
A heat sealer designed to seal food-storage bags may be used in place of a Mylar sealer. As with the Mylar sealer, use the lowest heat setting that will seal the material.
Keep the pinched balloon neck as flat as possible when sealing it to help prevent leaks.
Do not touch the insides of the heat sealer's jaws while the device is plugged in, as they may be hot enough to cause burns.
A flat iron or curling iron may be used to heat-seal Mylar balloons as well. If you have both, a flat iron is the better choice since its jaws are flat. While the iron warms up to its lowest heat setting, inflate the balloon with a balloon air pump, holding the neck closed as you would if using a Mylar sealer. Clamp the balloon neck between the jaws of the iron for several seconds; then release. If the balloon doesn't seal the first time, hold the jaws over the neck of the balloon for a few seconds longer, or turn up the heat setting.