Fill your canner with water and add pint jars, This made 5 pint jars for me. Bring to a simmer. In a small saucepan add the lids and bring to a simmer, then turn onto low.
Heat a big pot of boiling water and add the tomatoes in two batches, cooking just a few minutes to break the skin down. Drain and run them through your food mill to take out the skin and seeds.
Heat the oil in a large pan, then add the garlic and cook a few minutes.
Add the tomatoes after running through the food mill and cook at a low simmer 20 minutes or longer, depending on how thick you like your sauce. Add the basil and pepper flakes and cook another 5 minutes or so.
Remove the jars from the canner and carefully empty them (pouring the hot water back into the canner). Put them on a towel. Add 1 Tbs lemon juice to each jar. Fill jars with tomato sauce, leaving 1/2" headroom. Remove air bubbles and top off if needed. Wipe off the rims of the jars if needed. Put the lids on the jars, then the rings and hand tighten. Put back into your canner and process at a boil for 35 minutes.
Carefully remove the jars and place on a towel on your counter. You should hear the PING of the lids sealing fairly quickly. Leave out for 24 hours and check the seals by pressing down on the lids. (There should be no give.)
If any jars don't seal properly, store in the refrigerator.