For the method, it's easiest to go back to the source: Madhur Jaffrey's excellent World Vegetarian cookbook. Take it away, Madhur!
How to grate a tomato: When a coarse tomato puree is required, the easiest way to get it is to hold a ripe tomato over a bowl and grate it on the coarsest part of a grater. The skin stays in your hand while the puree collects in the bowl. Sometimes the tomato slides along the grater and does not engage with the sharp holes. In such cases all you do is cut a thin slice off the tomato. This will get it going.
We use a box grater, and it works great!