Open Outlook for iOS. The icon looks like this:
If you don't have it yet, download it from the Apple Store.
Note: Available for iOS devices using iOS 9.0 or higher.
Go to Settings > Add Account > Add Email Account.
Note: First time using Outlook for iOS? Just tap Get Started.
Type your full email address, then tap Add Account.
Note: If you're asked to select your email provider, you may have to set up your email as an IMAP account.