Elvis sung songs while he played on the piano all throughout the day, every day. He stated in interviews that there were songs he was working on and perfecting.
Elvis would play out the melody on the piano memorizing the intervals while listening to the note tones imbedding them into his mind. He sometimes would sing along with the melody played. He had an excellent ear for notes and chords and would play the accompaniment chords along with the melody. I'm sure he recorded his singing practice sessions and played them back for evaluation.
It is crucial for a singer to sing 10 to 20 songs a day always trying to improve his or her singing.
Reading well respected books on singing is done by all the great singers.