The Toyota coolant I have bought at a Toyota dealer is pre-mixed -- no water should be added to it.
There are many theories on how to correctly flush a radiator. You could Google "how to flush a radiator" and see a bunch.
Some techniques are to drain the old coolant, then add a gallon of radiator flush substance, top the cooling system up with tap water, run the engine until it gets hot, drain the water/flush substance and then fill the radiator with premixed coolant.
Once technique uses no flushing substance and only distilled water on the theory that tap water will leave unwanted residue in the cooling system.
I don't know which method is better. I have the coolant in our cars changed at a repair shop because I don't know of any place nearby to properly dispose of the old coolant.
Don't just dump out the old coolant on the ground -- and clean up any you spill. Apparently coolant tastes sweet and pets (dogs and cats) will lap it up and soon thereafter be rather deceased.