Take the route of going back up to Resting Grounds, west on the tram to Crossroads, take the top route so you can go up into Dirtmouth for the bench, west into the Greenway, down into Fog Canyon, then west into Queens Garden. The target is the room directly above the bench.
You can do it legit, or back up your save each time you hit a bench. The most annoying part is probably climbing up the right side of Crossroads, especially if it's infected (which it probably is since getting Monarch Wings will proc that event) from the flying bugs. Take it slow and you'll be fine.
I have zero interest in doing this achievement legit, save yourself a lot of annoyance. c:\Users\username\AppData\LocalLow\Team Cherry\. Copy the "Hollow Knight" folder somewhere else and you've backed up your save. If you lose the flower: exit the game, delete the existing "Hollow Knight" folder, drop your backup in, and continue.