Always ensure the butter is cold, straight out of the fridge. Cut into smaller cubes and add to the flour. Work with your fingertips and thumbs to take small amounts of the mixture and rub them together. Use just your fingertips - keep your hands out of the mixture to prevent it from warming and softening the butter. Keep rubbing in until the mixture looks like breadcrumbs. Shake the bowl gently to bring any lumps to the surface. Keep rubbing in until it’s all done. If the butter does get too warm, put the bowl in the fridge for 10 minutes.