I had some ringtones purchased from iTunes that I wanted to put onto my new HTC One. Googling for instructions found many hits, but many of those pages were a few years old, so the instructions didn't work anymore, or they didn't work with the HTC One, or were six-minute-long YouTube videos. So, I'm writing up some simple instructions that worked on my HTC One. (They will probably work with other Android phones, but I'm not promising anything.)
First, on your computer with iTunes, find the ringtones you want to transfer. These will be files with a .m4r extension. On my Mac, they were in the directory /Users/kdj/Music/iTunes/iTunes Music/Tones.
Next, you need some way to get the files onto your device's /sdcard/Ringtones directory. If you have a Windows computer, I think you can just plug the phone into the computer and it will be mounted as a USB drive, so you can just drag and drop the files. If you have a Mac, you can either use the Android File Transfer application, or use something like Dropbox.
Finally, you need to change the filename extensions of the files in /sdcard/Ringtones from .m4r to .m4a.
Then, on the device you can go to Settings > Sound > Ringtone, and you should see your ringtones in the list.