You must have a personal account in order to save content, and you must be signed in at the time. Please note that not all content can be saved (for example, you can only save individual chapters and not entire books).
The easiest way to save content is from the search results bar. Click the star at the top right, as shown below.

After clicking the star, you will get a popup notification confirming the save. You'll also have the option to apply or create a tag.

You can also save while viewing the article. If you scroll the page down slightly, you should see the Save star icon at the top right corner.
