the directions that come with Purigen are pretty simple:
1) step 1 is to soak in a 1:1 bleach/water solution for 24-hours
2) step 2 is to soak for 8 hours in water treated with Prime (or other de-chlor product) . . . 2 tblsp Prime per cup of water
3) step 3 is to soak for 4 hours in water treated with buffer (Neutral Reg, Acid buffer or Discus Buffer) . . . 2 tblsp buffer per cup of water
step 3 is only for FW tanks . . . don't know if it is truly "necessary", but they recommend it for FW tanks because bleach has a high Ph . . .
could be if you were keeping, say, an African cichlid tank, then step 3 is not necessary for you, but if you were keeping Discus, it would be a good idea