First of all, you have to pre-heat your oven to around 375 degrees centigrade.
Then, take a sauce pan and put it on the stove at low heat. Put butter into it and heat it for a while. Meanwhile, take the flour and whisk it well until it gets smooth. Now, slowly add the milk and cook for around 8 minutes until it gets thicker. You will have to whisk it continuously.
You have to mix the cream sauce with salt and nutmeg in order to add flavour to it. But, be careful while adding salt as its excessive use will affect the taste very badly.
Now, take a dish and coat its bottom with 1 cup tomato puree and cover it with 4 lasagna sheets. Then, you have to layer the lasagna with tomato puree, ricotta, basil, zucchini, mozzarella, Parmesan, cream sauce and lasagna sheets. You can make as much layers as you want. However, the layers should not surpass the capacity of the dish, otherwise the lasagna will come out of it.
Put the dish in oven and bake it for around 30 minutes. When it becomes golden and bubbly, take it out and let it cool down for around 5 minutes. Then, cut it into pieces and serve it.