You are two steps away from being the (web)master of your domain.
Step 1: Signup for a Developer Website
If you haven't done so already, signup for a new developer site using the "Create Developer Site" link at the bottom of this page. This will create a new website account using the base developer template.
Creating a free developer account automatically places your website into Sandbox Mode. In Sandbox Mode, your site will not expire, but it will also not be accessible without a CAPTCHA until you activate it as a paying account.
Step 2: Connect & Download Template Files
Once you have a Squarespace account with developer mode turned on, you can connect via SFTP or Git. The details needed for connecting are listed in the developer tab.
Connect via SFTP
remote path (initial folder): /site-name/
username: your account email address
password: your account password
Connect via Git
The template repository is available at /template.git for each site. Start by cloning the repo:
git clone https://site-name.dev.squarespace.com/template.git