Wash the tomatoes under a cold water tap
Cut the tomatoes in half or quarters (if large)
Place in a large saucepan with lid on
Put in the oven or on the hob at low heat for about 20 minutes, until the tomatoes are soft
When the tomatoes are soft remove from heat and use a food mill or blender to extract the liquid.
(You might need to run it through a sieve a couple of times, repeating the process with the material from the sieve)
Either store it in the freezer or resealable jars