In Squarespace Home Menu, click Settings, click Advanced, and then click Code Injection.
In the Header field, paste the code. Click Save.
This will connect your site with your Fundraise Up account so you can update your widget without reinstalling it each time you make a change.
Now go to the Campaigns tab in your Fundraise Up dashboard and create and customize your checkout. Here's an article explaining Campaigns in more detail. Once done, save the widget and click on Save and Test.
Head over to the Elements tab and make some buttons! Our Elements article explains the process in more detail, but in summary:
some Elements will give you HTML code which you can add to Code blocks in your Squarespace site,
and some Elements will automatically appear on some or all pages of your site as per their URL Control settings.