Peel tomatoes (blanch if you hate peeling). Use a food mill (or your hands) to remove seeds and separate juice. Removing seeds improves flavor & texture. Removing some juice will shorten cooking time.
Put tomatoes (will be mostly flesh) into heavy sauce pan (it’s OK if a few seeds sneak in). Add onions, peppers, garlic, oregano, basil, salt, and oil. Bring to a simmer. Cook until desired thickness.
Add 2 Tbsp lemon juice (or vinegar) to each quart jar (1 tbsp for pints). Fill hot jars with spaghetti sauce. Leave 1/2 inch head space. Wipe rims. Adjust lids. Process 45 minutes in boiling, hot-water bath* (35 minutes for pints).
Disclaimer: Always follow directions specific to your equipment and elevation for canning. Dispose of any home canned goods that show signs of spoilage which can include: bulging lids, leaking, corrosion, cloudy, mushy, moldy foods or disagreeable odors.