Open admin of your website and go to Plugins →Add New . Search for onesignal in search box. The search result will show related plugins where you can find the plugin named “OneSignal – Web Push Notifications”. Click on the “Install Now” button, Once installed click on “Activate” button.
Now we have successfully installed and activated the One Signal plugin for Push Messaging.
Now we can see a new option appeared on the left panel called “OneSignal Push”. Click on that and it will open the setup panel for One Signal Push Messaging. Click on the “Configuration” tab, and add the details of App ID, REST API Key, Safari Web ID, provided to you.
Follow the setup istructions as shown below. Once all done click on the Save button at the end of the page.
We can confirm that the setting is all working properly or not by going to the main website and check if we have the one signal bell icon 
appearing on the website.
