Wash the tomatoes and remove the stems. If the tomatoes have a large center core (as some big beefy heirloom varieties do), use a small paring knife to cut the core out now.
Allow the tomatoes to fully air dry, or pat them dry with a clean lint-free towel.
Spread the dry tomatoes out on a baking sheet or other tray that fits inside your freezer. Arrange them so they are touching one another as little as possible.
Freeze the tray of tomatoes for several hours or overnight, until they're frozen solid. This prevents them from sticking together in storage later.
Quickly transfer the frozen tomatoes into their final storage container (e.g. freezer bags). Don't let them thaw. Remove as much air as possible from the bag.
Store the frozen tomatoes in the freezer for up to one year. Best quality if used within 6 months. Use in any recipe that calls for cooking tomatoes, or in place of canned tomatoes.
To defrost frozen tomatoes, either allow them to thaw overnight in the fridge, at room temperature for about an hour, or under warm water. (You may not need to defrost them at all - could be used frozen whole or grated, depending on the recipe and called-for process.)
Once defrosted, the skins should slip right off. If not, make a small slit in the skin with a knife to help ease them off.