Step 1: Starting out
Step 2: Semi-Circumcision
As the picture shows, one can see the spokes of the tomato, this is where you will cut. See how it stands up? We cut the bottom up to give stability to the cutting process. Starting from the outer spoke, carefully cut around trying to avoid the seeds as much as possible.
If done correctly you should get two pieces like the second picture. There may be times when you have to cut the tomato into four or more pieces, just repeat the same process of cutting around the seeds.
Step 3: Surgery
Now go make some salsa or ceviche!