Work them through a sieve to remove the skins and seeds.
This is the real work of making the ketchup, and a strong sieve and kraut pounder really help get this done fast. Using a food mill can be very frustrating because the seeds may be too large to allow the mill to turn properly. It's easier to use a mesh sieve and a strong wooden spoon or pounder to smear the berries across the inside of the sieve and push the pulp through. You will still lose some of the pulp in the seeds, but you can get most of it this way.
Once you've removed the seeds and skins, return the pulp to the pan and heat slowly, adding your sweeteners, salt, pepper and spices. The sauce will become quite thick and sticky as it cooks. Let it simmer for another five to ten minutes while stirring constantly.