Separate hearing from production
Many singers treat intonation as a voice problem, but the issue is often an unstable reference or weak reproduction control. Train hearing and singing as separate steps first.
Use a curve instead of guessing
A real-time pitch curve shows whether you are generally sharp, generally flat, or start on pitch and fall at the end. This is much more actionable than simply trying to sing better.
Keep each drill small
Fix one note, one interval, or one phrase at a time. Move on only after three stable repetitions. Full-song singing is for checking progress, not for repairing errors.