The short version
For a baby's first flight, feed or offer a pacifier during take-off and descent so the ears equalise. Pack nappies for twice the flight time, a barrier cream, unscented wipes, muslins, a change of clothes, non-crumbly snacks and one new toy, all under the seat.
What happened
His first flight was Bengaluru to Delhi at six months, to meet his great-grandmother. I packed for a week-long expedition. I used about a quarter of it, and forgot the one thing that mattered: something to suck on during take-off.
Ears. A baby can't pop their ears, so the pressure change on take-off and landing is where the crying comes from. The fix is swallowing: a feed, a bottle, or a pacifier, started as the plane begins to roll and again as it starts to descend. On the way out I'd fed him at the gate and he had nothing to do during the climb. He screamed for eleven minutes. On the way back I held the feed until the engines spun up, and he slept through it.
The kit, refined over a dozen trips since. In the cabin bag: nappies for twice the flight time plus two, the yellow-lid cream, a pack of the unscented wipes, two muslins, a change of clothes for him and a T-shirt for me, snacks that don't crumble (makhana, a banana), the pacifier in its case, one small new toy he hasn't seen, and a zip bag for the inevitable. Under the seat, not overhead; you will not be able to get up.
Train travel, which we did more, is a different animal: longer, dirtier, and better. A lower berth, a muslin as a sheet, the same kit plus a small torch and a bottle of water for wiping down the tray. The thing the train has that the plane doesn't is other grandmothers, who will hold your baby so you can eat. Take the help.
Between six months and two he flew eight times and took the train six. The things we carried shrank each trip. The pacifier never left the bag.
The AAP advises offering a breastfeed, bottle or pacifier during take-off and descent so the baby swallows and equalises ear pressure. Airlines in India allow lap infants from a week old (check your airline), and the AAP recommends that where possible infants fly in a car seat, though this is rarely practical in India. Choking guidance applies in the air too: the CDC says children should sit up to eat and never eat lying down.
AAP HealthyChildren, Flying with baby: ears and air pressure. Source · CDC, Choking hazards. SourceThe verdicts
Pacifier in its steriliser case
The take-off tool. Also the landing tool. Also the reason the second flight was silent.
A backpack diaper bag with a wipeable changing mat
Hands free on stairs, under the seat on the plane, on your back on the platform. The shoulder-bag kind is for a different life.
A small steel snack box and a bottle of makhana
Doesn't crumble, doesn't melt, doesn't choke if he's sitting up. Our flight snack for two years.
A travel cot
Every hotel we stayed in had one or a bed we could floor-mattress. Carried it twice, used it never.
Product details are as we used them; the version on sale now may differ. Check the exact model and the manufacturer's instructions.