This achievement requires a comprehensive playthrough of the game where you basically do everything. The game shipped on PC in 2017 with 100% potential progress and content was added that increased that total to 112%. The Xbox version contains all of the content from the start.
After destroying the three Dreamers you can get into the Temple of the Black Egg in the Forgotten Crossroads. Interact with the statue next to the bench and your completion percentage will then be shown in your menu and on your save slot.
For progress towards 112%, this is what counts:Upgrades/Pickups
(1% each charm = 40% total):
(1% each = 17% total):
(1% each = 7% total):
(1% each = 3% total)
Colosseum of Fools
(1% each = 3% total
(1% each = 4% total0
Once you reach 112% progress, complete the game and the achievement will unlock during the post-credits summary screen. The Hollow Knight wiki
is a great source of information on anything specific.
These two maps helped me immensely, as well:Basic mapFully interactive map