How to heat-seal air-filled balloons.

1. Fill your balloon with a hand pump or eletric inflator.  A simple straw can be used to blow into the balloon as well. 
2. Once filled to the pressure you wish to have, you can remove the pump, and the balloon will remain inflated.
3. Now place the flat neck portion of the balloon.  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, but 4-5 seconds is normally a good amount of time. 
5. That's it! 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.

Perhaps you don't do many air-filled balloons, and don't feel the need to buy a heat sealer you will not often use.  No problem! You can use an iron to achieve the same goal. 

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 (be sure to 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.