Before beginning, generate the code required to implement your button: see here for info (part 1).
Login to your to your Squarespace account.
Choose an option
If you want the button to be present on a single page: Go to Pages > {select a page} > Page settings > Advanced > Page header code injection.
If you want the button to be present on every page: Go to Pages > Utilities > Website tools > Code injection > Header
Copy the head script and body script and paste them into the above section.
Then, navigate to the page you want the button to appear.
Click "Edit", then navigate to the section you want the button to appear.
Click on "+ Add block" > Code
Paste the button code
Click "Save", then preview your page to test the button.