Adding ring tones to your iPhone works exactly like adding other media -- add the ring tones to your iTunes 12 library, connect your iPhone to your computer and sync. The ring tones must be in the M4R file format for iTunes and your iPhone to recognize them as such.
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Launch iTunes and connect your iPhone to your computer. If you haven't already loaded your ring tones into iTunes, click "File," select "Add File to Library," navigate to the location of the ring tones on your hard drive, select them and click "Open" to add your ring tones to iTunes.
Click the "More" button, marked with three dots, and select "Tones" to show the ring tone section of your iTunes library.
Select the ring tones you want to sync to your iPhone.
Click and drag the ring tones, which displays a Devices column. Drag the ring tones over to the entry for your iPhone and release the mouse button to begin the sync process.