A total flush takes 3 times the volume of the pot a plant is in and takes a while to do. If you use RO water and don't overfeed your plants you shouldn't ever need to flush unless you overfeed and the plants are burning. If using hard water from the tap then flushing should be done on a regular basis to be determined by how hard and alkaline your water is.
First you water the plants until it is saturated then let it sit for an hour at least to dissolve all the salts. Then start pouring water through removing the runoff as it comes out. A shop vac works great for this if you can't put the pot over a sink or some other place to drain it. Check the ppm of the first runoff after saturation then as you go along. If just reducing nute level to stop burning then it may just take a quart per gallon of pot volume to lower the ppm enough to quit there. I had to do that recently with my plants and used 5L/16L pot after saturation with RO water to drop the ppm from around 1500 to around 700. Plants were fine after that.
If doing a complete flush of 3 volumes then flush 2 volumes thru after saturation then for the 3rd volume use 1/4 to 1/2 strength nutes at the pH you need to do the final flush. That's for plants that are to continue growing. If flushing to clean things out before harvest then just pH'ed water for the 3rd volume. The 1st 2 volumes don't have to be pH'ed as the third will set it right.
Good luck.