If you want to know what access tags are, go here
First, go to "My magazines" and then "Edit magazine".
Scroll down to "Access tags" and add the tags you want.
You also need to add this access tag to the users who should be able to access the magazine. Learn how to do that here
Also the magazine needs to be blurred, otherwise anyone can view it without having the access tag. Learn how to blur a magazine here
Learn how to add an access tag to a reader here