Take off and climb to cruise altitude
Fly along the route to destination
A contingency percentage is added to ensure that un-forseen events do not cause problems
Enough fuel to fly to an alternate airport where the weather has to be good
Enough fuel to land there with 30 minutes fuel left.
Three years ago a Boeing 777 landed at Heathrow (London) on the grass due to fuel problems caused by something that had not happened before - fuel icing at low level in warm air. This has now been addressed and the cause well understood. But the point is that the passengers were unaware that they had landed on the grass until they were told to “evacuate”. The landing was successful.