When you see the skins starting to come off the tomatoes, (in about 30 seconds or so), put them into a bowl of cold water and peel the skins off...they should come off very easily.
Then take your tomatoes and quarter them...put them into a large bowl or measuring cup.
(this is the point at which I sterilize my jars, to have them at the ready)
Take your quartered tomatoes, (2 cups worth at a time), and crush them in your large stainless steel pot using a potato masher. Keep adding and crushing until all of your tomatoes are in the pot. Now put on the heat and get that tomato mixture boiling on high heat for 5 minutes.
Get your mason jars out and at the ready. Put into each 500ml jar 1 Tbsp of lemon juice and 1/2 tsp of salt, (the salt is optional, but the lemon juice is NOT...you MUST use lemon juice...unless of course you actually want to get botulism)! Add the crushed tomatoes to within 1/2 inch of the rim. Wipe your rims clean, put on the lid and place in a water bath canner for 35 minutes. You must make sure your jars are covered with water...I find that with crushed tomatoes I have to add water to the canner several times. Have your kettle on the go and top up your canner with boiling water.