Turn off the heat completely, making sure there is at least one ladleful of cooking water left in the pan you cooked the pasta in. Gradually add the cheese paste to the pasta, stirring. If the spaghetti becomes too sticky, add a ladle of the reserved pasta water (it should be hot but no longer boiling, as high heat can cause the cheese to split and create lumps) and keep mixing. The cheese, cooking water and the starch will all bind together to make a creamy, emulsified sauce.
