Wash the tomatoes and bring a large pot of water to a boil. When the water is boiling, submerge the tomatoes for a few seconds to scald them and remove to a bowl of cold water to stop the tomaotes from cooking. Peel them, cut in half and remove seeds.
Weigh the tomatoes. For every 3kg of tomato flesh, you'll need 1.5kg sugar and 500ml water.
Put the water and sugar in a pan on the stove top. Stir over medium heat until the sugar has dissolved in the water, making a syrup. Bring the syrup to a boil, add tomatoes, and cook for 15-20 minutes, lowering the heat to a simmer. Be sure to watch the pot so that it doesn't boil over-it gets foamy! The foam can be skimmed off the top.
After 20 minutes, turn the stove off and let the mixture sit for a few hours.
After a few hours, re-heat, stirring continuously with a wooden spoon, taking care that it doesn't stick or burn. Bring it to a boil, then lower the heat and let cook for 1-2 hours, stirring periodically. The timing is a bit vague but you want to get to the syrup to a jelly set or when the temperature stablises to 104 C (219 F).
After this time, the tomatoes should have a shininess and transparent look.
Prepare your jam jars. There is a number of methods but I prefer to wash the jars, drain them well then place in the oven at 100C for 15 minutes Place the lids in a pan and boil for 10 minutes.
Pour the hot jam into the hot jars (use a jam funnel it makes it so much easier). Seal the jars immediately and then place in a pan with a cake rack at the bottom and pour over hot water to fully submerge the jars and simmer for 10 minutes (You don't want the jars to sit directly on the bottom of the pan you can also use an old tea towel).
Remove and cool. Using this method I have not had a problem with jam spoiling.