You simply download Decrypt9 from the GitHub page
here and then copy the .bin file to your SD:\luma\payloads\ folder. Should be named button_decrypt9WIP.bin where button is which button you wish to hold while starting the 3DS.
If you follow Plailect's guide
here it would take the place of hourglass9 and be called start_decrypt9WIP.bin
Be sure you also set the files9 folder and contents up according to playlet's guide as well.