UnderMine

So to transmute Relics, you must drink the Transmutagen potion while there are Relics lying on the floor (either Shop or ground). To get Careful Planning, you need to transmute at least 3 at once, which can be tough to get naturally.
Unfortunately you cannot transmute Relics that you have already picked up.
Fortunately, the game helps you out a bit.
After upgrading your bombs to destroy the blue colored walls at the beginning of a run you can unlock a character called "Hoodie" who can transport you to the first level of any of the maps that you have already reached. When you use the fast travel system, Hoodie will leave a bag next to you when you drop into the next map. when you open it, several relics will pop out, depending on how far you fast travel. Hoodie is available at the beginning level of each map.
Note: you must fast travel at least two maps - (Gold mines -> Hall or Dungeon -> cavern, etc.) otherwise it won't drop at least 3 Relics.
What I did was play through normally until I found a Transmutagen potion. Note: There is some luck required as you won't always come across a Transmutagen, but they appear often enough in shops or naturally. Once you have acquired the potion, just use Hoodie to fast travel at least two maps, open the bag and drink the potion. the Relics on the ground should all change and you should pop the achievement.
Good Luck!