^^^What they said^^^
Measure your phosphate levels and when you have a level sufficient for the GFO to work, then start it, also look at your tank so you can start getting a feel for how it reacts. You'll be able to tell a lot by what your tank tells you; sometimes you'll have algae with no measurable PO4 and sometimes you'll be able to tell when your PO4 levels are starting to creep up.
Either way, it is a good idea to start GFO before coral additions, so you don't shock them by reducing PO4 levels quickly. Also, don't try to completely eliminate PO4 as jsker said, your corals need some to grow.