The foil balloons used on pico flights do not have the most obvious fill point - so here is a quick guide.
Firstly a pic of what the neck assembly and valve within the balloon looks like:
Note the valve is made of two sheets of plastic that extend well inside the balloon. As the balloon becomes pressurised the sheets are pressed against each other sealing the balloon.
This is what I use to fill the balloon - 6mm clear plastic tube cut at 45 degrees to ease entry into the fill point.
This is where the fill tube enters the neck - push the tube an inch or so into the neck and hold it tight around the tube and gently fill away.
Note: If you insert the tube much further you may have trouble withdrawing it once the balloon becomes inflated. The valve becomes sealed with a very low differential pressure and clamps itself to the fill tube.
If you overfill the balloon its possible (but a little difficult) to remove some gas by pushing the fill tube well into the balloon and letting some out. If you do this feel for the valve within the balloon then hold the edge between the thumb and index finger as shown in the above picture and gently withdraw the fill tube. If you don't do this there is a danger of inverting the valve and pulling it out through the entrance.