1. Login into O365 and click the App Launcher.
2. Click the Admin App
3. Now you will be redirected to the admin page, Click “Show All”.
4. Click SharePoint
5. Click “More Features”
6. Within the app section click “Open” in Apps
7. Click App Catalog.
8. Click “Apps for SharePoint” from left navigation section.
If you don’t have the app catalog site setup you will be taken to a page where you can create the app catalog site. Once that is done you go back to step 7 and continue.
9. Use the upload link.
10. Click Choose files and upload the package.
11. Select the package file and click to upload the package file.
12. Now you will see uploaded package on the page.
13. Open the SharePoint site where you want to install the application (this has to be within the same domain where you have installed the add-in package.)
Click the Setting Icon and then click the Add an app link.
14. Click “From Your Organization” .
15. Click the app and the installation will start.
16. Click Trust It.
17. The installation will take 1-2 minutes. On the site content page you will see the app and from here you can start the application.