Heres an idea

listen to the source over a period of weeks, nay months if needs be until you find yersel humming the odd phrase, then add on bit by bit, to see if you can sing one verse , hurry not the learning unless you are gifted with total recall and of supreme intelligence, but imerse yersel totally in the piece

then when still alone

ditch the words and try
one verse at a time maybe in front of the mirror



Know your stuff thoroughly before attemping to perform it please   

respect the song and its author

I wanted to perform particular song before an audiencea few years back but luckily, I couldnt even start it, its taken me 3 years and I performed it perfectly last week.

Even the source artiste refuses to perform it, and he is a very famous folker indeed,

So the answer is 'practice' a discipline learned from 8 years of purgatory on the piano when very young

Good luck to one and all