Sorry about the pic quality but I was using a camera phone.
Hook the string in the crook of your thumb
![](http://img8.imageshack.us/img8/3140/1908091336.jpg)
Press your thumb tip against your middle finger
![](http://img16.imageshack.us/img16/3579/2008092029.jpg)
Fold your index finger over your thumbnail
![](http://img22.imageshack.us/img22/2089/2008092028.jpg)
And check this forum out. You can learn alot there about shooting with your thumb.
Hope this helps.