You don't say what country you are flying to or what airline you are using. Airlines vary GREATLY in what you can bring for baby. We flew often with our (now 15 mo) twins - beginning at 2 months. It's not too bad, really. Here are the things we learned:
1. Don't have more than 3 or 4 prepared bottles when you go through security or there will be a HUGE delay as some jerk decides to test each one for bomb materials. The local TSA people do not care that you are going on a long flight;
2. Even though it is more expensive, buy some of those 8 oz premixed formulas that you slap a nipple on for the flight. It is less hassle with security, less mess to prepare, and you don't have to deal with refrigeration;
3. If you are going to be on an extended trip and your usual formula is not readily available in the country you are visiting, order a case of the formula on Amazon.com and have it shipped to the location you will be staying;
4. Juggle your daughter's feeding schedule so that she will take a bottle upon takeoff and landing. Take-off isn't so bad if you can't get her to take a bottle if delays or whatnot messes things up, but we found a bottle upon landing was imperative for our guys;
5. Smile at the attendants, your seat companions, etc. It will put you in a better frame of mind, they will feel badly for their snarky flying with a baby thoughts, and you will get a lot more offers of help if she gets a little fussy;
6. We used diaper bags as our carry-ons. For the boys, we packed an emergency kit (benadryl, tylenol, etc. in a small ziploc), a small blanket, a couple of snap-on onesies or pjs, 1 diaper for every 2 hours of flight, disposable burp cloths, clean nipples for their formula, a couple of favorite small toys and their favorite stuffed toy. At 6 to 8 months we didn't pack cereal or other food because it saved space and they didn't need it for nutrition at that age. Packed right, it leaves room for your own carry-on items;
7. Do everything in your power to get an aisle seat. You will need it for walking if she fusses and to go do diaper changes;
8. Relax and enjoy your time with her on the flight. It will be good for both of you. We found traveling was actually a lot easier than we thought it would be when the boys were young.