If you have only one vendor then the code shouldn’t need changing at all, as if you only have one vendor then that vendor’s collection will have all products in it.

The gist that’s embedded in the article can be pasted anywhere you want on a product.liquid template, as the product object will be available. Where about exactly is up to you and where you want it to display. The portion below the {% endunless %} tag outputs HTML.

Yes backing up code is a good idea if you are unsure, you could also take on a shopify expert from somewhere. It should be a trivial task for them.