I've flown to UK from NZ with a 5 month old (2 x 12 hour flights) - it's pretty easy at this age.
Dress baby (and yourself) in really comfy clothes & take a couple of changes. They do have nappies on plane but they aren't great so take plenty of your own.
You should get a cot on the flight which is great for baby to sit, play & sleep but be prepared when the seatbelts sign is on you will need to have baby on your lap whether she is asleep or not. I also took cotton cellular blanket but they do have blankets on the plane.
I found other children on the flight all stopped for a chat/play with DD which she loved, small toys are easy to carry for amusement & when all else failed DD enjoyed watching a cartoon or 2 without sound (I cottoned on to this as she was peering at the screen used by the kids next to us - but this was on the 2nd of the 12 hours flights).
Depending on the time of day she may sleep for a lot of the flight. At 5 months DD slept for majortiy of our first 12 hour flight which was a night flight.
I was BF and did this take off & landing. If not BF then have drinks ready for baby to have at these times - staff are pretty good with parents & babies in my experience & will take your meal tray quickly & bring it to you first etc.
I also found a sling useful in getting to & from the plane (rather than a buggy). I had sling & small backpack - be warned some airports (ie Terminal 3 at Heathrow) don't bring the buggy to the plane - I was well stuffed up by that after 26 hours of travelling with sleeping 2yo. Thank goodness I had the sling with me then!!!!!!!!!!
Re jet lag I can't say I really noticed this with 5 month old - she wasn't weaned & was BF every 4 hours or so anyway so not much difference there. I did need support from others with DD, as I dealt with my own jetlag which is more of an issue.
I've always travelled with DD's carseat (as checked in luggage) and airlines will carry them for no extra charge. Great as you have your own seat at the other end.
Happy travels!