Method 1: Best for Round Tomatoes
Core the tomato and cut in half crosswise with a serrated knife to expose the seed chamber. Gently squeeze out the seeds, using a finger or a small table knife to help empty the chambers. Lay the seeded tomato halves, cut side down, on a cutting board. Holding the serrated knife parallel to the cutting board, cut the tomato halves horizontally into slices that are as thick as you want your dice to be. Next, cut each stack of tomato slices into strips as wide as you want your dice, and then slice these strips crosswise into dice.
Pros: You use the whole tomato; good for beefsteak tomatoes, which have a lot of delicious inner flesh.
Cons: Takes a little more time; the stacks can be a little awkward to slice.
Use a small table knife or a fingertip to flick out the seeds.
Cut the tomato into strips and then slice crosswise into dice, as you'd cut an onion
Method 2: Best for Plum Tomatoes
Cut a thin slice from the bottom to create a flat surface on which to stand the tomato. Cut wide strips from the top, curving down to the bottom, to separate the flesh from the inner seed core. Cut all the flesh away in this manner, leaving the seedy core of the tomato; discard the core. Cut each strip of flesh lengthwise as wide as you want your dice to be, and then cut these strips crosswise into dice.
Pros: Fast; good for tomatoes with big seed chambers and less inner flesh, such as plum tomatoes.
Cons: You don't use the whole tomato; not great for really small dice because the flesh is usually at least 1/4-inch thick.
Cut the flesh away from the seeds in wide, petal-like strips.
Cut the strips lengthwise and then crosswise into dice.