What you need:
- 3ds/2ds with US/EU/JP regions only. New or old version is fine.
- Pokemon Picross downloaded (no cost)
Warning: This will overwrite your old Pokemon Picross save, but you can back it up first with Nintendo's official backup feature on home menu (no homebrew needed). https://en-americas-support.nintendo.com/app/answers/detail/a_id/70/~/how-to-back-up-save-data