How do I add personal MP3's from my computer into my Music app on my iPhone?

I used to be able to just drag and drop my own personal music collection onto my iPhone so I could listen to it. Now that I've subscribed to the Apple Music service, I can't get my own personal music into my Music app on the phone.

I'd like to listen to music Apple Music doesn't have on their service, such as music that I've written and recorded, video game soundtracks, and artists whose albums are inexplicably missing. This would be especially useful on roadtrips.

I've tried googling the answer, but everything assumes I want to do something with an Apple Music subscription. I don't. I want to listen to my own music that's not on Apple's service.