You can actually just toss the ferritabs directly into the pool – they dissolve quickly. In the photos we have a pool that was recently filled mostly with a truck, however the first 3′ were with iron filled well water. You can see the progress before, during and after treatment and just how much iron the filter was able to take out.
If it is a freshly filled pool, use 3-4, let the filter run 24 hours, backwash and repeat.
If you just topped off water I typically add 2-3, run the filter 24 hours and repeat if either the backwash was super orange or if there is any tint of color to the water that I can see.
PS – if you have stains on the surface of the pool pre-treat with Natural Chemistry StainFree to lift the stain, and the FerriTabs to remove the iron from the water.