The needle bottle tends to be much better than the bottle most liquids come in. Its precise design makes it a lot easier to avoid making a mistake as the needle tip allows you to fill many clearomisers without the risk of getting liquid into the air flow hole or spilling liquid as you pour.
Fill your needle bottle with your favorite e-juice. Angle the clearomiser tank so that the needle is facing away from the core. Allow the liquid to run down the side, filling the tank. Don’t fill past maximum capacity, as you may damage both, clearomiser and battery. Follow the markings on the side of the tank to know when to stop filling up.