I go about it the other way around:
First get the song "in your head" by listening to it a lot.
Then make sure you understand the rhythm by clapping along, tapping your toe or heel until the rhythm is also "in your head".
Then look at the organization of the song, which chords does it use in what order, what are the patterns, where are the repeats, are there different chords for the chorus, etc.
Then begin to play the song slowly a little bit at a time while humming or using Da de das or some such substitution, when you have the bit up to tempo, move on to the next bit.
When you can play it all the way through, at tempo with Da de das start using the words. By then it should be fairly simple to learn the words.
If you are learning a song for fun play it until you don't make any mistakes. If you are playing it in order to perform play it until you can't make any mistakes.
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