How do I heat-seal an air-filled balloon?

1. Inflate your balloon. 
2. Once filled it is okay to remove the pump as these are self-sealing to temproarioly hold in the air.
3. However, you still need to heat-seal to ensure the air does not escape at all. Do this by placing the flat neck portion of the balloon on the heal sealer.  You want to seal the area between the hole where you filled it and the neck of the balloon.
4. Hold down the heat sealer.  You can set the heat-sealer so that the red light turns off when the seal is complete.  You will have to play around with this at first, but 4-5 seconds is normally a good amount of time. Too long, and the neck will melt off.  Too short and it will not properly seal. 
5. Let go of the heat sealer, and remove the balloon. 

Here is an example of an air-filled balloon.  This was filled with air (not helium), heat-sealed, and then attached to a cup and stick.

1. Follow the same steps 1 and 2 above.
2. Protect your iron from the balloon by wrapping the bottom of the iron with kitchen foil where it will touch the balloon. DO THIS BEFORE THE IRON IS TURNED ON. Find a non-flammable hard-surface to press the iron and balloon against.  You may wish to cover this with kitchen foil as well.  
3. Start at a low-setting of the iron, and press for a few seconds.  You may need to practise a few to find the correct heat level and amount of time to seal the balloon.