You have to add it To raw plastic. Meaning stir the hardner in before you cook it. Be sure to mix it really well. You might could get by with just buying medium plastic dePending on how hard of a bait u want. Might could save u some money by just gettin medium also. Ha but then again if u truly need the hard you will wish u bought hard instead of mixing in that much hardener to medium! Hope this makes sense!??