Lasagna is a convenient meal to make before the start of a busy week because you can prepare a large amount and sock away individual portions in the freezer for later cooking. If you forget to do this and instead cook the entire batch first, you’re not doomed to eat 20 servings of lasagna in the next couple of days -- you can still cut up and freeze the leftovers.
Cut up the lasagna into single portions no more than two hours after you take it out of the oven.
Put each serving into a separate freezer-safe container, or wrap it in freezer- and microwave-safe plastic wrap.
Put the servings in the freezer. If you wrapped each in plastic wrap, put the servings in a bag to keep them all together once the lasagna has frozen to the point where it isn’t so soft. If you stack them all in a bag while they’re still soft, the packs on the bottom will become crushed from the weight of the ones on top.