In Hollow Knight: Silksong, Hornet has a number of skills at her disposal, which deplete the Silk spool beneath the health bar. The spool can be lengthened over the course of the game by collecting Spool Fragments, a pair of which will add one segment.
This Hollow Knight: Silksong guide is a definitive listing of all 18 Spool Fragments and their locations.
Bellhart
Frey will become available after completion of My Missing Courier, allowing you to buy the Spool Fragment for 270 Rosaries.
Blasted Steps
In the tall shaft at the top of this region, go about halfway down the left side. You will stumble upon Grindle here and he will let you purchase a fragment for 680 Rosaries. Requires Faydown Cloak to reach.
Choral Chambers
Complete The Lost Merchant and interact with Jubilana at Songclave to be able to purchase a fragment for 500 Rosaries.
Cogwork Core
At the very bottom of this region, proceed towards the right till you descend even further down. Fight an enemy and get through the gate, and continue right to the very end.
Deep Docks
Starting in the shaft at the bottom left of this area, enter the passage midway up the right side. The upper section of this large room will require you to platform all the way to the right and double back to the left to reach the fragment.
On the far right of Deep Docks, head to the shaft shaped like a boot. Requires a Simple Key and Clawline to get a door open.
Grand Gate
Platform your way to the top of the moving mechanism in Grand Gate to get to a fragment.
Greymoor
At the top of the larger shaft situated between Sinner’s Road and Craw Lake. Requires Cling Grip.
After locating 15 of the Lost Fleas and interacting with the Flea Caravan, join them on the journey to Blasted Steps.
High Halls
Using Clawline and Cling Grip, get to the very summit of the leftmost shaft in High Halls.
Memorium
On the left side of this region in the widest shaft is an area bristling with thorns, that you will have to navigate till you reach a little lower than halfway down for the fragment.
Grotto
From the elevator in the large main shaft, head right into the tunnel above the exit to The Marrow. Proceed upwards from this area and bust down the wall beyond the traps there.
The Slab
Beginning at the bottom of The Slab, begin ascending the left side of the tall, narrow shaft with Cling Grip.
Underworks
Starting in the shaft above the path to Wisp Thicket, head right into the lowest passage. Fight your way through the enemies till you can collect the fragment.
Entering the Underworks via Whiteward, bear right and continue descending to the very bottom of this large space.
Weavenest Atla
Square in the middle of Weavenest Atla, go to the middle passage that heads about halfway left. Begin ascending using Cling Grip.
Whiteward
Take the elevator all the way down, then double back up using the passageways in order to recall the elevator up. Wait in one of the middle passages for the elevator to pass you, then go down to the very bottom of the shaft for the fragment itself.
Complete Balm for the Wounded and interact with Sherma to get this fragment.
Those are all 18 Spool Fragments in Hollow Knight: Silksong.
