It depends on what kind of hair extensions you are wearing. For clip on hair extensions, it is hard to do french braid. For tape on and keratin hair extensions, I think you can treat the hair as usual. Just be careful to make sure the bond or tape is invisible.
I am assuming that you know how to make a normal braid. In French braid you mainly start with 3 small sections of hair from the top of your head. Whenever you cross 1 section over the center section you include small amount of hair from the sides. This is how the French braid is made. For more details visit following site.
French Braid
French Braid