Introduction
Short cut
Create a short cut to Google Scholar on top in the sidebar.
Open the sidebar in the Safari window (the the right of the previous/next page navigator). make sure that the tab for Favourites are selected (the one that looks like a book rather than a pair of glasses). By default (at least in December 2019 and for Mac OSX 10.14 [Mojave]) there are two default entries besides the Favourites folder: Google and Bing. To create another “top” link, highligt an existing top link (i.e. Google or Bing) and right-click, then select copy from the menu that appears. Then just paste, either by just pressing the key-combination cmd+V, or via the Safari menu:
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You will then get a duplicate of the item you copied. Mark it with the cursor, and then right click to change both the name and the url associated with then new top item. Google scholar for instance.
Google scholar
I set this up a long time ago, and do not remember how, but once you have it, continue. But you can use this funny youtube introduction on Google Scholar.
Library
Once you have signed up with Google Scholar, you will also have your own library available under your account. To add a reference to your library, search for it using the Google scholar search engine. Once you have located the reference, simply click the small star (with blue outline) just below the reference, and it will be added to your library.
Return to your library, and you should see the article you just saved. To categorize it, click on the checkbox for the reference, then click the Labels icon. If you already have a label under which you want to save the article, just click that label alternative. Alternatives you can create a new label and then add then reference to that label. You have to press Apply to actually put the label to the reference.
Reference text
To see the reference as a text, click the citation logo (2 quotation marks) next to the star. You can do that directly from the search results, or from the entry saved in your own Google scholar library.
Add Library
If you are associated with an academic (or similar) library you can link the access of your library to the google scholar search engine. In my case, being employed by Stockholm University, I found the instructions here