If you refrigerated the broth from the pork butt roast, you will have a bunch of fat that congealed at the top of the container. Use a spoon to scoop that out and throw it away.Or, if it's your kind of thing, add it in with the liquid in the next step. Or save it for some other thing. I imagine this can be used for cooking fat. In a medium pot, mix the water, broth, and corn starch and bring to a boil. Then reduce heat to simmer
If you want thick soup, simmer for a while to evaporate some of that water.
Add the solid ingredients and simmer for 5 minutes so the veggies soak up some of that liquid.