For Diced Tomatoes:
To substitute fresh diced tomatoes for a 14.5-ounce can of undrained diced tomatoes, use approximately 1 1/4 cups of diced fresh tomatoes and 1 cup of liquid. If your recipe calls for a 28-ounce can of undrained diced tomatoes, use about 2 1/2 cups of diced fresh tomatoes and 1 cup of liquid. Use any liquid that seems appropriate for your recipe, such as water, broth or tomato juice.
The skins get tough though when cooking fresh tomatoes, so either blanch and peel or make sure to pick them out as they cook. Depending on the variety of tomato, you probably won’t have to add any liquid either–most are pretty juicy on their own. I usually quarter mine after peeling and then squeeze as much of the juice out as possible just with my hand before tossing into the pot, otherwise it takes forever to cook down. (and then the fresh juice makes a fine base for a “healthy” bloody mary!)
For Tomato Paste:
Dehydrate (in the sun, oven, dehydrator, whatever works) smallish variety of tomatoes (grape, cherry or roma), then use a food processor to blend into paste. The tomato skins that are removed for stewed tomatoes can also be turned into paste by plopping them into a food processor as well.
For Fire Roasted Tomatoes:
Quarter your tomatoes and roast in the oven or on the grill (can add a foil packet of smoking chips with holes in the top over the hot coals for extra smoky flavor), then removed peels and dice.