First wash and dry the tomatoes, and then peel them. To peel them you can either chop them up into quarters and push them through a food mill, or run them through a blender and push the puree through a fine sieve.
Once the tomatoes are peeled and deseeded, put them in a covered pot and bring them to boil at low heat for around 10 minutes.
Uncover the pot and cook for another few hours. You should stir every 15 minutes or so to ensure an even consistency. If it is sticking too much to the pot, lower the heat and stir more often. Keep stirring until the mixture has been reduced by about a third.
Transfer the paste while hot to airtight jars.
Keep in a cool dark area like at the back of a pantry cupboard for up to 1 year. After opening refrigerate and use within a week.
NOTE: If you are concerned about Botulism you may need to add a couple of tablespoons of lemon juice or half a teaspoon of citric acid in the jar. This can be done before or after transferring the tomato paste.