By now you don't. You have to remove the battery and get the phone clean within a minute or two. You could try an alcohol wash. Remove the battery, Out the phone into a bath of rubbing alcohol (the stuff sold in drugstores). Swish it around for a few minutes - you want to get everything out of/off the phone. If the alcohol gets dirty, dump it and start with fresh alcohol. Then let the phone dry at least overnight.
Alcohol isn't water, it absorbs water, and draws it out of the phone.
It may be too late. If there's corrosion on the motherboard already, even if you completely clean and dry the phone, it'll corrode further in even humid weather. But for a couple of bucks worth of alcohol it's worth the effort. I've saved more than one phone that was dumped into a toiet.
(Wear gloves when handling the phone. One tiny cut that you can't even see and you can get deathly ill handling what's on the phone. I always used triple gloves - cheap insurance.)