You don’t need to be exact when layering a lasagna, and a lot will depend on the ingredients you use and the size of your pan.   Here’s the way I layer a classic lasagna.   I start by covering the bottom of the pan with the sauce; this prevents the noodles from sticking and burning at the bottom. Then I add a layer of noodles. Next is the ricotta filling. Followed by a sprinkle of mozzarella. Repeat the noodles, ricotta, and mozzarella. Finish with lasagna noodles, sauce, and the last of the mozzarella.   If you’re using meat or vegetables, layer those in after the ricotta cheese. But again, it really doesn’t matter as long as you start with sauce and noodles.