@stillrivereditiCongrats on getting your first ecommerce store up. I think squarespace is a website builder type of service, right? If that’s the case from what I’ve seen those providers will have established plug ins or methods to use gateways.
On the projects I do, I generally start from scratch with an empty folder and start making php, html, css, and JavaScript files. In that scenario there are usually going to be a lot more choices to make and also infinitely more ways to do things wrong.
For services like I think this is, they typically follow a principle of intuitive design called “build your software in such a way as to prevent your users from breaking something”. So I would look on the squarespace site for what kinds of authorize.net integrations others there are using. If you are a small business, as all my clients thus far have been, you want to minimize your PCI scope and make sure that you keep track of your compliance. You will almost certainly fall under the SAQ A scope with whatever method you use, so go to the PCI website and pull the SAQ A document to familiarize yourself with those requirements.
So my big tip is to get started. Start the process of getting your stuff set up. You may run into some bumps. Post them here and that will give me something to do while I wait on my pancakes in the morning. I will look out for your progress. If you have more specific questions as you get into this post those too.