Boxster Overheat and Coolant Waterfall

[aaf006]

So after swapping the thermostat and water pump and went on a decent 20 min drive. Temperatures were quite low but then shot up again. No coolant boiling/ waterfall this time however. On the old pump, there was one plastic piece that was broken off but Did not see in the bay. How might one do the reverse flush?

Once again I appreciate everyone's help!