I looked up tomato paste in Putting Food By.
My sister used to make this and it was really time consuming but really good.
If you are going to can it, use a boiling water bath, hot pack only. Use
only 1/2 pint jars.
Carefully wash, peel, trim and chop the tomatoes saving all the juice
possible (4 to 4 1/2 pounds of tomatoes will make about 4 1/2 point jars of
paste). In an enameled kettle bring the copped tomatoes to a boil, then
reduce heat and simmer for 1 hour, stirring to prevent sticking. Remove from
heat and put the cooked pulp and juice through a fine sieve or food mill.
measure, return to the kettle, and for every 2 cups of sieved tomatoes add
1/4 teaspoon citric acid (or 1tablespoon white vinegar), and 1/2 teaspoon
salt. Reheat, and continue cooking very slowly, stirring frequently, until
the paste holds its shape on a spoon - 2 hours or more. Hot pack only, in
1/2 pint jars. Ladle hot paste into clean hot jars, leaving 1/4 inch of
headroom Adjust lids, and process in a B-W Bath (212 F.) for 35 minutes.
Remove jars; complete seals if necessary.