You go on your contact list
you select the contact you want to add a ringtone (press the screen)
after select OPTION and scroll down the list !!! You will found the option for ringtone, picture id ... etc !!!
Good luck ! and sorry for my bad english !!
I had already done assigned individual ringtones the way you suggested. It still doesnt work. According to the contact the ringtone is set but when the contact phones the ringtone is the standard ringtone for all other calls. I think the phone is broken. How do I proceed.