Not a big deal

After you poured it in, if you have not started engine yet, you can just unscrew the radiator drain plug, let fluid run away; fill in tap water to rinse it out; refill with 50/50 coolant. That's it.

If you have already started engine, that means the windshield fluid has been circulated in the system, you would need to do more - unscrew the radiator drain plug; plug a garden hose into the radiator inlet (upper), let water run; start engine running for a few minutes, until you see the drain fluid is clear (at that time, your engine system has only pure water. Stop engine, let radiator dry up. close the drainage, fill in 100% coolant to radiator until full (because there is still 50% water inside engine so now you get 50/50 mix in entire system. Done.