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 looks like:

Note the valve is made of two sheets of plastic that extend well into 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 enter the fill point easier.

This is where the 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. If you insert the tube much further than that you may have trouble withdrawing it when the balloon becomes pressurised. The valve becomes sealed with a very low differential pressure.

if you overfill the balloon you may need to remove some gas - in order to do this push the fill tube well into the balloon and let out some of the gas. Now hold he valve 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.