How to load ringtones on iPhone 6s

I had 2 ringtones on my iPhone 4, one was a steam locomotive whistle which sounded for all incoming phone calls.

The other ringtone was assigned specifically to one person, the Beach Boys' song: Barbara Ann.

The steam loco whistle was downloaded from a 3rd-party website. Barbara Ann may have been also, but I really don't remember.

I backed up my iPhone 4, then restored everything to my new iPhone 6s. Everything copied over except the ringtones.

The 'Barbara Ann' ringtone is in my iTunes Ringtone folder on my computer. I had to find and download the steam loco whistle and it is now sitting on my Desktop. (FYI, I have an Apple iMac desktop computer.)

The iMac won't allow me to drag and drop the steam loco whistle from either the Downloads folder where it once was or from the Desktop where it is now.

The first Apple Support person I talked to couldn't figure out how to get the steam loco whistle into the iTunes Ringtone folder. The second one I talked to suggested downloading it directly from the website to my phone while viewing the website on the phone. I tried that. Maybe the problem here is with the website, but I don't know. When I press the button to download the iPhone version of the steam loco whistle ringtone, it just hangs, nothing happens. When I press the button to download the MP3 version, the screen says 'Thank you for downloading', another screen appears and the ringtone begins to play. Seems like it worked, right? When I look in the list of ringtones in Settings, the steam loco whistle isn't listed. So where did it go? Did the download really work?

Since the Barbara Ann ringtone is already in the iTunes Ringtone folder , you would think I could just click on 'Sync Tones' and and then sync the phone. But when I try that, I get a message box that reads "The iPhone XXXmy nameXXX is synced with another iTunes library on XXXmy nameXXX Computer. Do you want to erase this iPhone and sync with this iTunes library?" "Erase this iPhone"????? Now that sounds scary. I'm not going to try that.

So, I have one ringtone in iTunes and one on my desktop and I can't get either one onto my 6s. Can anyone help?

Oracle