i'm planning on flushing the system in the next 2 weeks after this heat wave passes because the coolant smells fishy... and its orange

..
so ive been doing research and ive concluded these are the correct steps (correct me if im wrong)
1. drain coolant via drainrooster or lower hose if the rooster is too difficult.
2. follow directions on prestone flush bottle, bleeding the system with each drain and refill.
3. drain all the rinse water out and add 50/50 prestone
what im wondering is.. if i use plain water to rinse the system and then drain via the radiator, wont there still be alot of water in the block/heater core when adding 50/50 coolant? i havent found much info on that so if someone could clear that up...