I put it in loose between the pads. Then I hook it up to the tank in my sump, and point the output tubing in to a nearby bucket, and plug in the pump. Once the water coming out is clear, I divert the output tubing back into the sump, and throw away the water. It will be several gallons worth. Replace that with freshly mixed saltwater and you'll have a mini-water change done as well.
According to RowaPHOS, you should test the output solution for PO4. It should read zero, and it it doesn't, close the ballvalve to slow the flow and test again. It should trickle out slowly and read 0. Once the output is over zero, the media is used up and needs to be replaced again.