There are 3 secrets to a smooth silky carbonara sauce. Firstly, beat the eggs well so they they are smooth and properly mixed together. Secondly, add some pasta water to the sauce as the starchy liquid will make the sauce delicious and silky. And thirdly mix the eggs into the pasta off the heat. The residual heat of the pasta will cook the eggs, but if you leave the mix on the heat, you'll end up with lumpy scrambled eggs.