I found it was the most simplistic to do on the very 1st song of at the beginning of Career in the 1st Venue (The Cavern '63), entitled, "Twist and Shout." Now, as you'll see when you play there's a bar of difficulty that rates the songs and seeing as it's the 1st setlist, they're all fairly easy, even if you struggle at these type of games. When you select the song just hit the "Back" button to flip to "Lefty", chose a fairly easy difficulty, I went with Medium and after I flipped the guitar around and played the song, I ended with 92%. You'll truly feel like Paul McCartney by the time you're 10G richer :D
Good Luck!!
P.S. If you're naturally Left-Handed, well this really wont be that much of a challenge now will it :D