Ha! Did lots of buns before - i think im an expert, but isnt it rather difficult to explain here??

Anyway, here I give it a try:
For French Twist.
Twist your hair. Tucked your twisted hair up - hide the end of the twist between head and the Twist if you dont want your Twist has tail. Pin from below, as if you sew along the hair. Now push the head of the pin (which should be facing downward) upward, and push into the Twist.
For round bun.
Twist your hair. Bun it as if you want to make a traditional bun. Pin from left or right, as if you sew along the hair. Now push the head of the pin to opposite direction, and push the pin into the Bun (if you start from left, your pin's head will end up on right side, and vice versa).
For both, two-legs pin is better than one-leg chop-stick-like pin. Zig-zag-legged-pin is better than straight-legged pin.
Dont get it?

Why dont try googling? Maybe they have one with pictures on it? Or you can buy pin (ex.: from Chomel) and ask the girl to show you the trick. Or else, ask hairstylist to do your hair, and make sure you learn the trick from him/her.