If you are used to visa-free travel or visa-on-arrival, India works differently, and the differences catch people out on the day they fly rather than the day they apply.
Apply at least four days out
Apply no later than four days before your intended arrival. This is not a same-week decision, and it is definitely not an airport decision.
Your passport needs at least six months of validity at the time you apply, not at the time you travel. That distinction matters if your passport is close to expiring.
Print it
Carry a printed copy of your Electronic Travel Authorization, and before you leave, confirm the status reads GRANTED.
An application that was submitted is not an application that was approved. Checking the status is a thirty second job that prevents the worst possible version of this trip.
Expect biometrics on arrival
Your biometric details are captured when you land at Indian immigration. Nothing to prepare, but it is worth knowing so the stop at the counter is not a surprise.
Everyone applies separately
Each traveler needs their own passport and their own e-Visa application. That includes minors. There is no family application, and a child on a parent’s passport is not covered.
Two things they can ask you for
You need a return or onward ticket, and sufficient funds for your stay. Have both available rather than assuming nobody will ask.
The yellow fever rule Americans and Europeans often miss
If you are arriving from a country affected by yellow fever, you must carry a Yellow Fever Vaccination Card. Without one you risk six days of quarantine on arrival.
This catches travelers routing through parts of Africa and South America, including on a single connecting itinerary. It is about where you have been, not where you are from.
What you will need to upload
A recent front-facing photograph on a white background, and a scan of your passport bio page. Getting the photo right first time is easier than resubmitting.