Essentially to flush, you run water thru your plant until the water coming out the bottom is clear.

So, if you have a 3 gallon pot, you would run 3 gallon flushes. A 5 gallon pot, a 5 gallon flush.

The number of times required is dependent upon how many times you need to flush to produce clear runoff water.

So make sure your drainage is adequate!

Hope this helps.

Oh yea...

Some people flush PRIOR to flowering as the ferts needed changes in this state. Others ony flush a week or two before harvest.