You don't flush prior to flowering (unless you overdosed the plant on nutrients). Stop using your nutes 2 weeks before harvest time to ensure that all chemicals are out of the plant. Flush the plants with fresh water 2 weeks prior to harvesting them. Make sure you flush at least 3 times the capacity of your pot. So a 3 gallon pot would be flushed with 9 gallons of fresh water. From that point water your plants with pure water, absolutely NO additives should be given to the plant. This is especially important if you have been using chemical fertilizers.