I have already the Facebook business page and Instagram account. But how to connect them to each other?
To connect your Facebook Page and Instagram account:
1. From your News Feed, click Pages in the left menu.
2. Go to your Page and click 
3. Click Instagram in the left column, then click Connect Account.
4. Toggle Allow access to Instagram messages in Inbox on or off. When on, it provides access to people who manage your Page to view and respond to Instagram messages.
5. Click Confirm.
6. Enter your username and password and click Log In.
7. Important! After you connected the Instagram Account to the Facebook page don’t forget to Add business tools
