Step by step guide to move / point SquareSpace domain to netlify app?

I have recreated an existing Square Space site with Svelte and now host the svelte version under Netlify. The Square Space site has a custom domain. I am not the owner of the Square Space site, but I have access to the settings.
I have googled and read many of the SquareSpace forms about this, but there isn’t a step by step guide.
I have tired to add a CNAME record to SquareSpace but apparently the defaults are there so I can’t add another.
Can someone break this down for a non-networking person.

  1. Go to Netlify site settings → Domains → Add custom domain
  2. Based on if you’re adding an apex domain or a subdomain, you might have to add different records: Configure external DNS for a custom domain | Netlify Docs

Those are the only 2 steps that we can provide, as to how you can do the changes on SquareSpace end, that’s not something we can answer.