For a soft-boiled egg, lower the egg into gently boiling water with a slotted spoon. Allow three and a half minutes for a medium-sized egg and four minutes for a large egg. The yolk will be runny and the white just set.Cook for a further minute if you like your soft boiled eggs a little firmer. For a hard-boiled egg, start the egg in cold water and bring up to the boil.Once the water is gently boiling, set the timer for between 7-10 minutes depending on how well cooked you like your eggs.The longer you boil the egg, the firmer it will be.Once cooked, plunge the hard-boiled egg into plenty of cold water for one minute.or a soft-boiled egg, lower the egg into gently boiling water with a slotted spoon. Allow three and a half minutes for a medium-sized egg and four minutes for a large egg. The yolk will be runny and the white just set.