How do I backup to OneDrive?

There is no option for an iPhone to backup to OneDrive; iCloud or a computer using iTunes are the only choices for backing up an iOS device. You can save files (e.g., from Microsoft Word app) to OneDrive.

You may want to make a suggestion to Apple that they offer more backup options via Feedback - iPhone - Apple

You can backup/sync/upload some content, but not a broad range like iCloud offers. And no complete system backup as iTunes and iCloud can provide.

Use OneDrive on iOS - OneDrive

You can't. At least not in the way you back up on iCloud. What you can do though is back up individual data such as documents and photos and then delete them from iCloud to free up space.

OR just back up to iTunes on a computer instead.

To my knowledge, you cannot backup your entire phone to OneDrive. What you can do is auto backup your photos to OneDrive.