When I first found out about Steel Soul mode I thought it would be incredibly difficult, especially getting the Steel Heart achievement. Doing everything in the game without dying sounds terribly hard. But I soon learned that the game isn't as evil as it first seems. You don't actually have to do everything despite having to reach 100% completion. The trick is that since the Grimm Troupe update, the Lifeblood update and the Godmaster update the maximum completion in the game is 112% and this means that we can get away with just ignoring 12%!

Of course this guide is also applicable in normal mode but the main purpose of it is to explain the easiest way of getting the Steel Heart achievement. I don't want to encourage people to just flat out ignore game content, especially on your first playthrough! This game is great and you should experience all of it!

Obviously, since we want to make this task as easy for ourselves as possible, we choose to ignore the argueably most difficult parts of the game:

• White Palace
  
• The Trial of the Fool
  
• The dream bosses (dying to these wouldn't end your playthrough in Steel Soul mode btw)
  
• The Flower quest
  
• Crystal Guardian (both fights)
  
• Grimm
  
• The entirety of the Godhome

What exactly are we skipping here that would normally be required for completion?:

• The Kingsoul Charm (1%)
  
• Trial of the Fool (1%)
  
• Mask shard from flower quest
  
• Mask shard from Crystal guardian
  
• Mask shard from the Seer
  
• One other mask shard of your choice
  
• The Awoken Dreamnail (1%)
  
• The Seer's Ascension (1%)
  
• Defeating Grimm (1%)
  
• Banishing the Grimm Troupe / Defeating Nightmare King Grimm (1%)
  
• The Godtuner (1%)
  
• The 4 Pantheons in the Godhome (4%)

The game counts one full mask (4 mask shards) as 1% so all in all we are skipping the maximum of 12% with this plan.
Some of you might not find the Crystal Guardian to be difficult (especially since he's been nerfed a bit in the Hiveblood update) but since we're already skipping one full mask there is no reason to fight him unless you want to skip a different mask shard instead.
It's also worth noting that as long as you don't mind the overall difficulty, you can do the White Palace and the dream bosses without danger of dying.

These are the things you'll have to do for 100% completion:

• Collect all other mask shards
  
• Collect all vessel fragments
  
• Collect all other charms
  
• Acquire all three spells and their upgrades
  
• Beat all the warrior dreams
  
• Acquire all abilities and nail arts
  
• Upgrade your nail 4 times
  
• Defeat the following bosses: Gruz Mother, False Knight, Hornet, Brooding Mawlek, Mantis Lords, Soul Master, Dung Defender, Nosk, Broken Vessel, Watcher Knight, Uumuu, Hornet (second fight), The Collector, Traitor Lord and Hive Knight
  
• Absorb the three Dreamers
  
• Obviously also defeat the Hollow Knight in order to finish the game

For some more precise details of what exactly gives how much completion, check out this amazing guide:
https://steamproxy.com/sharedfiles/filedetails/?id=936948247
This also includes a detailed route through the game. You can follow this route while ignoring the parts I mentioned earlier and still get the 100% completion achievement.

If you disagree with my judgement of what to skip you can make your own plan! Use the guide linked above for reference and choose whatever you want to skip yourself. In a sense, this guide is just to let you know you can skip the most difficult parts while still acquiring 100%. The parts to skip here are just based on my opinion after all.
I hope this still helped you learn something and good luck with your future playthroughs!