To make a Mary Berry pie crust, you'll need plain flour, butter, and a pinch of salt. Start by sifting 225g of plain flour into a mixing bowl, then rub in 100g of cold, diced butter until the mixture resembles fine breadcrumbs. Add a tablespoon of cold water and mix until the dough comes together. Wrap the dough in cling film and chill it for 30 minutes before rolling it out to fit your pie dish. This method results in a flaky and buttery crust perfect for both sweet and savory pies.