1) Sing the first line over and over till you can do it without the lyric sheet, and without thinking about it.

2) Sing the second line over and over till you can do it without the lyric sheet, and without thinking about it.

3) Sing those two lines over and over without the lyric sheet, and without thinking about it.

4) Add the third line.

5) Rinse and repeat till you've got the whole song. Give the last part extra attention because that will be the part you've sung the least.

Only way I know to do it.