Flushing usually involves poring approx 3 times the amount of water in your pot as the pot size to rinse out any unwanted build ups in the soil that might contribute to problematic results from things like salt build ups, or the build ups of other nutes that may block out the up take of needed nutes by the plants. So, if you have a 1 gal pot then you would flush with 3 gallons of water, or at least until you see clear water coming out of the bottom of the pot. Some even take it to the sink and run water over the soil until this affect is accomplished, but if you have hard water or a water with a high PH you may want to reconsider, or just give your plant a gallon of ph'd water with a very light amount of nutes after the sink water run.
Hope this helps some.
Adieu