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.
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.