If you're going on a European airline she will need an infant/ extension seat belt. She will have to be in this whenever the seat belt sign is on.
Do not lie her on the floor/ use a bouncer (won't be allowed) by your feet etc the carpets are incredibly dirty and this is incredibly dangerous (I've seen toddlers by pax feet).
If the crew aren't put with carts and the seatbelt sign is off take her for a walk.
Take karvol, you should be able to buy it in boots at the airport. Get the crew to put some of the thicker paper hand towels in a double plastic glass with a small amount of boiling water. Drip the karvol into it. It'll help clear her sinuses if she struggles.
Take a pillow in hand luggage if you can.
If it's a small plane (single aisle) it's highly likely the extra legroom seats will be next to the doors and you will not be able to sit there anyway due to the infant. If it's a large twin aisle plane there may be bulk head/ extra legroom seats where you can put a bassinet (some airlines carry them, some don't, usually these seats need to be pre booked- just for future info).
If you're lucky and there are spare seats and you are able to spread out, she'll still need to be on your lap in the infant/ ext belt whenever the seat belt sign is illuminated.