My approach might not be the right one but here are my thoughts.
If you can't just try different rolls over the chords of the song, then you might not yet be ready to take on this task yet. I know it sounds simple, the backup that's being played (and I don't think it is all that complicated) but you need some skills before you can get there.
I used to think that if the backup was just tabbed out, then I could figure it out but really, even if someone tabbed something out, then all you would be able to do would be to try and play that memorized backup. You really need to be familiar with chords and with rolling over chords.
To do this, you need to start at the beginning. Ben's lessons are great for this. He teaches you simple songs where you learn your chord shapes and your rolls and you play these over and over and over again until they start to become automatic.
Then you can take a song like this and just fiddle around and find something good to play. But you have to start at the beginning. Learn some songs and play them 1000 or times. You don't need to practice rolls, you'll do that playing the songs.
As for this song, you can play in open G so you would be playing a 4 finger A or you could capo on the second fret (and spike on 7) which would then be open A (so A would be open, D would be a 3 finger C shape and E would be a 2 finger D shape).