You can reach the Slab fairly early in Hollow Knight: Silksong once you enter the Choral Chamber area after beating the Last Judge and going through the Underworks. You can find it by continuing to go ...

On the left-hand ledge of this section, Flintslate is waiting to be claimed. Grab it, and then double back to your nearest Bench so you can slot it in right away. Once equipped, Flintslate in Hollow ...