Personalise ringtones using tunes from music library

I am not tech savvy and could benefit from foolproof help in creating ringtones from my music library. My daughter has given me her old iPhone 4s to replace my old Nokia. On my old phone I had different Beatles tunes as ringtones for my family. Sad but true, they all have names in the titles of songs. I lost these tones when I moved my contacts etc over to the iPhone. I have researched how I could create the files to download onto the phone. I tried to follow the steps in a MacWorld article but cannot find a way to change the file extension as I do not have the "show in finder" menu option. The phone is using ios 8.4.1 so I think it is right up to date. Any help greatly appreciated.