With fresh pasta sheets purchased from the main supermarkets they come with waterproof separates between each sheet (often 6 in a pack).
You place the cold sheets in a baking dish and pour on boiling water and leave for 3/5 minutes. You then carefully drain the water off and lift each sheet in turn as you place in your lasagna dish, disposing of the separator sheets. Place two on the bottom, place some of your cooked lasagna mix on top, then pour a layer of the cheese, butter, flour and milk mix/sauce, after boiling, on top. Repeat by placing another two sheets of pasta on top of that layer, then the lasagna mix, sauce, then the final two sheets of pasta on top, with the rest of the sauce. Place under a grill to lightly brown, before heating it all again in the oven at 180c for five minutes.
You then have a lasagna to serve!