There is no left or right-handed way to play the guitar/banjo/mandolin ( unlike playing tennis, or whatever), because it is a two-handed instrument! This subject has been discussed at length before. I feel this desire to play "left-handed" comes from the "first encounter" syndrome! By that, I mean when anyone ( left or right-handed) picks up a guitar, banjo, mandolin or even a sitar for the first time, all they can really do is run there fingers over the strings (i.e they won't be able to form chords or play melodies) and they will naturally do that with their best hand. But, it is a two-handed instrument. Learn to play in the othodox position - you will enjoy the instrument a lot more that way.