Wash the tomatoes and make two cuts to form an X on the tops and bottoms. There are two ways to remove the skins. You can blanch them in boiling water for one minutes, or steam them in a steamer basket until the peels start to come lose. Either way works.
Once the skins start peeling off, carefully remove the skins, taking care not to burn your hands. Then blend the tomatoes.
Place a strainer on top of your pot and strain the tomatoes directly into the pot. This is just to strain out the seeds. Then turn on the flame.
Add the vinegar and sugar to the tomato puree. Bring the mixture to a boil, then reduce to a simmer and let the puree simmer down to half. Taste for acidity and sweetness and adjust. Then add the laal mirch and cumin seeds. Simmer for another 5 minutes. The ketchup should have deepened in color and become a thicker paste like consistency. It won't be as thick as store bought ketchup but it should not pool back quickly when you run a spoon through it.
Let it cool completely before transferring to an airtight container and refridgerating.
To make it even more smooth you can run an emersiaon blender through it once it's slightly cooled.