run copius amounts of water through it, or you can buy and item like clearex something to help wash away the salts. I grow in soil to, and I hardly use the clearex unless I have a problem and need to do an emergency flush during the grow. End of harvest I quit feeding 2 weeks out, and start flushing at 10 days out, and again at 7, and at 4 and then I let it fully dry. You want to add enough water that you are getting good runoff out the bottom. If in a 5 gl pot, I would like to see at least a gallon of run off each time...hope this helps.
Alot of people, especially organic growers, don't even do a flush...and it's kind of hard to screw anything up. If you do it like I said don't freak when your bottom leaves start really yellowing and falling off, this is what your going for, youhave washed out all nutes, and forcing the plant to use up it's reserves.