How bad is “bad”? How good is “good”? What is bad and what is good?
A study was done at Stanford University and it was discovered that the only people who are truly tone deaf have brain damage. I worked with a brain-damaged karaoke singer and was able to get him to match pitch, carry a tune, and to sing in tune with accompaniment, so that may not be an absolute “unfixable” condition. My having the curse of perfect pitch was quite helpful in resolving his issues with hearing and identifying how his voice fit in with music.
Without some analysis of the singer, it is a bit of a guessing game. I would assess several factors, but first would determine the goals the singer has: To sing to perform and is the goal to be an amateur or a pro? Once that is established, I would have the singer warm up and then sing a song. Then the analysis would be done. Within the many components of musicianship, on a scale of 1 to 10, how do those rank? Without analysis, there is no way to know where to start. How is: pitch, rhythm, tonality, accuracy of intervals, perception and performance of melodic lines, etc.? It is a long list.
Then, I would evaluate if there were any issues with the 21 most common problems of singers which include: articulation, breath control, use of the voice (as opposed to abuse of the voice) and many more. I will usually know which ones are problems, after having heard a song sung.
There are 18 components of singing and performance mastery. Artistic imagination and objectivity are but two of the 18.
I do not do a “one size fits all” kind of instruction. I do not have singers work on things they have already mastered. I don’t have a mental image of how a singer should sound. Everyone is different from everyone else. No two singers sound exactly alike and imitating singers can be dangerous to the health of the voice. You do not fit into an imaginary mold. I do not waste singers’ time with useless or outdated vocal exercises. Any vocal exercises I use are proven, since there have been over 200 Grammys won by people who use them. Each one has a purpose. Some are used to build the voice, while others are used to maintain it or, in a state of advancing in quality and agility. This includes range, which will eventually be extended to its fullest, but without causing strain or other problems.