I have only just learnt to do a french plait but once you get used to picking up a new piece of hair (and combining it with the hair already there, haha) its quite easy to get the hang of.
Here is how I learnt:
1. Get three pieces of hair just like you would plait a normal plait but start right at the top of your head and only get small pieces of hair not your whole head. If that makes sense.
2. Plait those pieces of hair a few times, once you have done that each time you are about to plait a strand of hair get a bit of hair from you hair not in the plait and at to the strand of hair you are about to plait. Do this with EVERY strand of hair until you get to the end when you have no more hair left to add.
3. If there is still enough room to plait some more but you have no hair left to add to the strands you are plaiting then just plait them normally untill you get to the end.
4. Tie end with a hairtie and clip some layers back or your fringe back if needed. And spritz with hairspray.
This is how I do my french plaits and I find it easy. I hope this helped.
I'm 30 and I still don't know lol!