How to properly use the 'More tag' or another way to show teaser content

I’m trying to figure out how to get the ‘More tag’ and/or short codes for levels working.

Currently I have about 6 categories to post in. Such as “Sports”, “Education”, etc. I also have a protected category called “eArticles”. If I want to protect a sports article then I just put it in both the category of “eArticles” and “Sports”.

Instead of doing this, I’d like to let every protected post have some teaser content, and then make someone be one of our access levels in order to read the full story. I have read about the ‘More’ tag but I’m not entirely sure how to use it. The manual/youtube videos don’t show usage and just putting [more] before the content I want protected doesn’t work right. Could someone explain to me the proper way to do this? Please and thank you!