I flew regularly when each child got to 9 months.
The older one had a carry on pack with paper, stickers, crayons, cards, games, lots of little things. This was before phones on planes. The pack was started at 2 years onwards.
The baby would be on my lap.
I'd take a large pashmina a good one that was huge and lightweight but warm and I'd tuck the baby into me. It's cold on long flights. She'd sleep mostly.
Always time a feed for take off and landing. We always flew with local airlines as the planes were smaller and the Pilot made his ascent and descent gradually so the children on the flight didn't cry. This was one of my main reasons for flying from a smaller airport.
We also paid for priority boarding as it was easier with a baby in a buggy. The buggy gets folded and left as you go into the plane. It's then the first thing they unload the other end.
Have a means of wearing your baby. Sometimes you need to carry both of them.
My worst flight was with a baby and a 3 year old landing at Gatwick (never again). No buggy in sight, a long long walk at 4am with a young child needing a wee. They couldn't hold on and we had pee everywhere. I hadn't realised it was such a long way to the toilets from the plane even though I'd asked the crew for help. I have never flown with children from a large airport again.
My tip is to get them to wake and use the loo just before descent even if they are sound asleep.