Wash and cut tomatoes into quarters. Roma tomatoes won't need to have juicy insides removed, but if using a juicier variety, pull the jelly inside out with fingers. Add tomatoes to a pot.
Heat cut tomatoes on medium low heat until they begin to break down and become soft. Stir occasionally to avoid sticking. Once they release more moisture the heat can be increased to a medium simmer until tomatoes are reduced by half.
Preheat oven to 225 degrees. Lightly rub olive oil over a large, rimmed baking sheet such as a half sheet pan.
Work cooked tomatoes through a food mill or mesh strainer to remove skins and seeds. Pour strained tomato pulp onto oiled baking sheet.
Spread tomato pulp evenly over baking sheet so that there is a uniform thickness less than 1/4 inch but thicker than 1/8 inch. (Too thin and it will burn.)
Bake at 225 degrees for 2-3 hours, using a bench scraper or rubber spatula to scrap up and redistribute tomato pulp evenly every 20-30 minutes (closer intervals towards the end of cooking) until thick and darkened in color. You will notice a reduction in size as the moisture evaporates and it becomes a paste.
Cool tomato paste. Divide into 3-6 tablespoon portions by scooping spoonfuls onto a cutting board or spooning into ice cube trays. Freeze solid, then when frozen the portions can be collected into a zip lock freezer bag and stored in the freezer for up to 5 months.