Hi Marta and thanks for your question. It's a good one!
When freezing lasagna, I'd ideally suggest using an appropriately sized plastic or 'Tupperware' box, as these help the lasagna hold its shape a little better than bags.
However, if ziplock bags are all you have, not to worry. Just don't put the still-warm lasagna still in there... instead cover the leftovers with kitchen paper until it's completely cold (and therefore more solid).
Then you can put the lasagna into a bag, maybe chopping it into portions first and bagging each individually (this will help you work out how many you can serve when defrosting it next time!).
PS: Since you love lasagna Marta, you might enjoy my Pastastic pasta cookbook. This includes a full section of easy-to-print lasagna recipes and is supplied with a free exclusive Lasagna 101 guide. Ciao!
