I would add a few more points: I forget the exact term, but be sure to engage in purposeful practice, always challenging yourself with a song just out of your reach. Start as easy as necessary and work up.
It would be helpful to do some songs that you can get the full score for, so you can do concrete validation of your correctness. However, be sure that they are the full, true score; grab a Guitar book of Beatles songs, for instance, and they're likely to be simplified in a way that won't help on this task.
This right here is worth repeating. Never pick something you're certain you can do. Go for things just out of your reach. Pick things that you're going to have to strain to do. Do this again and again, and every time you get a bit better.
Occasionally, attempt something far beyond your reach, just to remind yourself of what you're striving for.
It would be helpful to do some songs that you can get the full score for, so you can do concrete validation of your correctness. However, be sure that they are the full, true score; grab a Guitar book of Beatles songs, for instance, and they're likely to be simplified in a way that won't help on this task.