Getting the tomato is the hardest part, really.
Slice off the stem part. Next slice it evenly almost to the bottom, but not quite.
If you cut it all the way through you are gonna hafta do it the old fasioned way.
Now rotate the tomato 90 degrees and do it again.
Try to keep them even.
Now turn the tomato on its side so that the uncut part is facing your non-dominate hand. Then with your dominate hand slice in even strokes all the way through.
Tada! Tomato cubes.
After you slice off all of the cubes you can just cut up the part that's left however you want to. This saves a lot of time and effort. Saves on headaches as well. Enjoy ^_^