In a small pot, place the lids and cover with cold water. Bring to a simmer, turn off the heat and leave the lids in the hot, but not boiling, water, in order to activate the rubber sealing band.
In a large pot, place the jars and cover with water. Bring to a simmer over medium heat. Adjust the heat to low and leave the jars in the pot while you prepare the recipe. Drain them just before filling with tomato sauce.
Feel free to add aromatics of your choice to this plain tomato sauce, such as garlic, basil leaves, bay leaves, oregano, etc. Do not add salt when cooking the tomatoes. The salt is added directly to the jars during the canning process.