Completing Silk and Soul in Hollow Knight: Silksong isn't actually as complicated as unlocking it. There's quite a bit of stuff you'll have to do to unlock Act 3 before you get this quest (which I go through in the guide linked above), but once you've received it from Songclave's Caretaker, you'll have one final task to complete with the quest itself.
Silk and Soul is pretty painless as quests go, unless you haven't beaten Groal the Great, in which case, forget everything I said—it's extremely painful, though more on that in the Seeker Soul section. Below, I'll explain all four soul locations and where to find them.
How to complete Silk and Soul in Silksong
The main goal of Silk and Soul is to find four souls located throughout the map in some hidden locations so you can build a trap for Grandmother Silk. Here's where to find each:
Silksong Maiden Soul location
You can get the Maiden's Soul from the Chapel Maid near Bone Bottom. You'll remember this NPC as the first one you met in the game after you emerged from Moss Grotto. Simply head left from Bone Bottom over the Moss Grotto shaft to find her above the chapel and get the Maiden Soul.
Silksong Hermit Soul location
Another easy one to get—this comes from the Bell Hermit below Bellhart in the tunnels where you found the Silver Bells. Head to the Bellhart Bellway, go right to the far end [[link]] into the hidden passage, drop down into the area, and go left, hopping across a gap, to find him and get the Hermit Soul.
Silksong Seeker Soul location
Now, if you haven't already done the Groal the Great boss in Bilewater, I am so, so sorry for what you're going to have to go through to get this. Genuinely, I would not wish Bilewater on my worst enemy. You'll have to defeat Groal the [[link]] Great in his arena at the top of Bilewater and then head into the hut behind his arena in Bilehaven, where you'll find the Seeker Soul.
Looking at the map above, route wise, you'll have to climb to the middle path (that corridor between those two benches on the map), then drop down at the end to unlock that lower bench as a checkpoint. Then, I'm sorry to say, you'll have to climb up and left all the way to Bilehaven where the second bench is (after the boss) to even attempt the fight—it's genuinely a very bad boss runback.
Silksong Snare Setter location
This last item is located in Weavenest Atla in Moss Grove. If you haven't been there yet, it's behind the bug-faced door to the right of the area—use the Needolin to get it to open. Head inside, go down the lift, then jump up above the door on the right and hop over the lift shaft. Carry on a little way and you'll find a hidden passage in the roof that you can jump up. Check the clip and map above for reference.
All you need to do now is bring all four bits back to the Caretaker to complete the quest and unlock the alternate Soul Snare quest for beating the final [[link]] boss and allowing you to enter Act 3.
