How do I download my own music for my ringtone

See if this helps, Use tones and ringtones with your iPhone, iPad, or iPod touch - Apple Support

This User Tip will also help a great deal. Create and manage ringtones - Apple Community

There are also a number of apps in the App Store for creating ringtones.

This should help:

Drag the music file (MP3) you'd like to use as a ringtone into the "Ringtones" folder. On your phone, touch Settings > Sound & notification > Phone ringtone. Your song will now be listed as an option. Select the song you want and set it as your ringtone.

sapper wrote:

This should help:

Drag the music file (MP3) you'd like to use as a ringtone into the "Ringtones" folder. On your phone, touch Settings > Sound & notification > Phone ringtone. Your song will now be listed as an option. Select the song you want and set it as your ringtone.

I don't see how that's going to work as iPhone ringtones are not MP3 files. iPhones use only .AAC files with a file extension of .m4r for ringtones. I don't believe just dragging a file to the "Ringtones" fold (I assume you mean on a computer) will perform the required conversions. See the user tip ChrisJ4203 referenced for more details.