Hi, we used one last year when DD was about 19 mths. They are just like a car seat and come with the 5 point harness.
The only thing I would say is that even if you request one direct with VA before flying, there is no guarantee that you will get one as I think they only carry a couple on each plane.
I was expecting it to be already fitted when we got on the plane but it wasn't. I asked the FA and she had to go to a storage locker at the back of economy to get one (we were upstairs in PE).