Do you mean this song?
https://www.youtube.com/watch?v=1I7zMKptjRs
I work out my strumming patterns by ear. Aldrine Guerrero has a good explanation of how to do it on UU+
Now, if this is the song you are talking about, you will need to change the chords half way through the strumming pattern, so if you are playing it in the key of C, it would go (C)DDU(Am)UUDU, or you could strum at double speed. Either way works just find with the song.
The strum that works for me (with a low G) involves plucking the G string before strumming, so it goes (C)GUXU(Am)GUXU, with X representing a chunk. Not saying you have to do it this way, but you might want to play with that sort of thing if you are feeling adventurous.