First, open the application you want to prevent from automatically starting up (you can skip this step if the application is already kept in the dock). I’ll be using Microsoft Word to demonstrate, but the process is nearly identical for almost all apps. Is one particular app bothering you every time you log in? You can end the annoying behavior easily by changing the app’s settings. This will open a Finder window for you to choose the application you want, and it will be added to the list of apps in System Preferences.
Alternatively, if you accidentally remove an app that you want to continue opening at startup, you can choose the small plus button instead.
