If you’re using a smaller bag, like a commuting laptop bag, most items will sit upright and be easy to access, so you don’t need to worry much about organization. When using a large travel backpack, I like to use packing cubes for clothing and items I won’t need mid-flight, and then I pack them at the bottom of the bag. Pack the things you’ll need during the flight last so they’re on top and easily accessible. If you have a side access panel on your bag, place your easy access gear on the side, and make sure the panel is facing up when you stow the bag under your seat.