If the water is already near the rim of the bowl, do not add chemicals. You will cause an overflow of caustic liquid. In that scenario, turn off the water supply, bail the bowl, and call a plumber immediately.
Leave just enough water to cover the drain hole. Pour 1 cup (250g) of caustic soda flakes directly into the toilet bowl, aiming for the drain opening. caustic soda for blocked toilet
You cannot pour caustic soda into a toilet full to the brim. Use an old cup or scoop to remove most of the standing water into a bucket (dispose of this dirty water outside or down a sink after the toilet is fixed). If the water is already near the rim