How do I make the songs in my library a ringtone?

If you have Garage Band on your phone, you can from what I read in this article, use it to create a ringtone. See if this works for you: https://9to5mac.com/2017/11/10/iphone-ipad-custom-ringtones-make-ringtone-garage band/

I don't know that there is the capability to do it on an iPhone alone.

Use tones and ringtones on your iPhone, iPad, or iPod touch - https://support.apple.com/kb/HT207955

Create and manage ringtones in iTunes. - https://discussions.apple.com/docs/DOC-12051 - including handling ringtones in iTunes 12.7 and later

Along with GarageBand there are other apps in the App Store that will crop a song segment to 30s or less and change the extension. However, you’ll need a computer with itunes to actually sync those ringtones to the iOS device for use as functional ringtones.

Interesting!