If you want to do a comprehensive flush.
Drain the system from the bottom hose, disconnect the top and use a hose to flow water through it. When all the crap stops flowing through it and clear water comes out, put your hand over the bottom exit and fill it up, then empty it. Continue to do that as the hose doesnt clear everything.
THen place the hose into the bottom coolant hose going towards the block, and flush that out, then do the expansion bottle.
Fill it up with water and coolant system fush, run it for a bit (at least until the fan comes on and off 3 times). Then drain, do the same and fill with the proper D type coolant.