Flush water in through the thermostat, which you have removed, and let it come out through the opening for the circulation pump, where you have taken off the line from the lake water intake.
If you have a heat exchanger flush it separately.
Do not flush water through the lake water pump.
Turn the circulation pump by hand before you finish flushing as you may get sand left from the casting process in the circulation pump.
Flush for about ten minutes with high water flow.
Remember to have the plug out before you start filling the bilge with 10 minutes of water coming out the circulation pump.