you just pour it in the toilet bowl. The excess amount of water is what makes the toilet flush as opposed to using the water stored in the tank.
Toilets have a goos neck below the bowl, this is what keeps the smells from the sewer from coming up and stinking up your bathroom. the goosneck keeps a water trap between the sewer and the bathroom, so when you introduce a few gallons of water in the toilet bowl (by either flushing or pouring water) the weight of the water pushes the water through he goose neck thereby flushing the toilet.
Hope that helps.
PS this is also how you can unclog a toilet, pour about 3 gallons into the stopped up toilet and it will flush.