Thickening your ketchup: The ketchup will thicken as it cools and sets in the fridge. But if you'd like it to be extra thick, after step five, transfer the ketchup back to its saucepan. Bring it to a low simmer. Mix 2 TBSP of corn starch into 1/4 cup of cold water, and slowly whisk the mixture into the ketchup, whisking until you get the consistency you're after.