Drain, add tap water to cool, and peel them off. Cut them in half and discard any hard white parts. Puree them.
Let cool a little and transfer into a jar. Refrigerate.
During cooking you test for doneness by dropping a small quantity on a cold plate and checking the consistency. Remember that when cold, it will thicken a little more. It is better to undercook it than to overcook it. If you undercook it, you can always boil it a little more!
Cooking time may vary. It will depend on the type and size of saucepan, how juicy the tomatoes are and other factors.