To create a Buy Button for either your products or collections in Shopify you'll first need to make sure that you have Buy Button added as a sales channel before the you can create a Buy Button. This can be done by following the instructions on this page.
Once Buy Button has been enabled as a sales channel we can proceed to customize and create the Buy Button and retrieve the code to add it to the editor. These steps are all described in this guide by Shopify.
Regardless of whether you're creating a Buy Button for a single product or a collection once you've hit the step where you've clicked Copy Code we're ready to move to the editor.
Select App Integrations
And then select Third-Party Integrations
From here click the Add a third party integration button and find Shopify in the popup that appears:
You should now see a field where you can enter the code from earlier:
After adding the code press back, this will take you back to your list of added integrations. Toggling the switch will enable the ability to add the button to your pages.
Navigate to the page you want to add the button to, and click the add a section button. You will be able to see your button under the category "Section Integrations".
Once you've added it to the page that you want the button to appear on click publish and it will now appear on your live site.