Look its a long hard road. I think the first thing you have to ascertain is “Is this person tone deaf?” can the student hear that they are singing out of tune? If the student has no idea they are off then thats the first thing to be done is to help the student hear that they are off. They may never get there. If the student says “ yeh I’m way off” then there is hope. start with one note and patiently correct the student until they get there. If they can hear they are off then they’ll get there. Then slowly move a semitone and back, then a tone and so on. Sing with the student to reinforce the melody when you can and drop out. Let them sing alone for a bit then jump back in when the trainer wheels fall off and they wobble.