Reach Crystal Peak
Obtain the Lumafly Lantern
Enter Crystal Peak from Dirtmouth via the lantern-lit route, or use the Desolate Dive/Descending Dark shortcut if available
Progress through Crystal Peak to the area above the Mantis Claw section
Defeat the Crystal Guardian
Continue upward through the platforming section
Reach the top-right chamber
Collect the Crystal Heart ability from the pedestal