I love Victorionox serrated knives for most veg prep! But if you haven't got that brand any small serrated knife will do the job. Or a very sharp straight edge knife.,
Cut tomato in half, top down so you cut through whee the stem was.
Make sure the top (stem end) is to the left. (if your left handed then to the right)
Put tomato down flat.
Make slices through the tomato, look closely at the pic and you can see them. Don't cut all the way through stem end as this holds it altogether!
Another shot of cutting down...
One cut across. Again not all the way so it all holds together.
Now cut downwards!
Till you get to the end piece and then chop this up.
Done! :-) the size of your cuts will dictate thR size of dice. So for large dice do less cuts more spread out!
Let me know if this makes sense and how you get on! More than happy to add pics and info if this isn't clear!... :-) happy chopping