First of all, the tomatoes are cut in half, placed on a baking sheet with a lip and roasted at 180°C/350°F for an hour to an hour and a half (less if they are small, but most of mine are big). You can add seasoning, garlic and herbs to the cut tomatoes, but I tend to leave them basically quite plain and add flavourings to the sauce when I use it later. This burns off a lot of the water in the tomatoes and you can do a lot at one time. At the height of the season I cook three baking sheets at once.
After they have been roasting for the hour, remove them from the oven. There will be a lot of water in the sheet.
If you're happy with the consistency of the sauce, you can proceed to the canning stage. If not, you can boil the sauce down a bit in a saucepan until it's thick enough.
I've never found it necessary to add lemon juice to the sauce, but if you prefer to do so, add it now. Pour the sauce into sterilised glass jars and cover with lids and rings. Put jars into a pressure canner and process for 30 minutes.