Stop flushing
Turn off the water supply valve behind the toilet
Put on rubber gloves
Use a flange plunger
Make sure the plunger covers the drain opening completely
Push down gently first to remove air
Plunge firmly up and down several times
Repeat until the water drains
Pour hot water into the bowl if needed
Add a small amount of dish soap if needed
Wait a few minutes
Plunge again
Use a toilet auger if the clog remains
Insert the auger into the drain
Turn the handle to break up the blockage
Pull the auger back out slowly
Flush once the water level is normal
Clean and disinfect the area
Call a plumber if the toilet still will not drain