Here's my routine for learning a song that I want to perform:

1. I transcribe the words (from whatever source) by hand into my song book.
2. I read/sing them continuously several times from the book, looking away and testing out my memory.
3. When I feel I've got the words in my head, I take up the guitar, turn away from the words and sing to the window or the wall, only looking at the words when I genuinely can't remember them.
4. I use the mental image of the handwritten words in my head as an aide-memoire in remembering.
5. Once I've got the words and tune and guitar parts in my head, I get into "performing" mode and perform it to myself as though I were in front of an audience - over and over and over again...
6. I usually get Mrs. F. or some friends to have a private listen.
7. I'll do the first public performance at an informal session or a singaround, and see how it goes as a musical entity.
8. After 40 years, I don't need feedback on whether my performance was good or not - this is not arrogance on my part - I know by now (age 64) whether I'm performing it to a reasonable standard or not!

Depending on the complexity of the song, the whole process might take several weeks, because it's surprising how quickly a quickly learned song can slip the memory. If you treat it as a slow, maturing process, the song stays better in the memory and "plays" better to an audience.

Just my 10-penn'orth for aspiring performers...